Application of graphene derived nanomaterial in microwave absorption has been limited by the issue of excessively high dielectric loss. To address this issue, instead of resorting to burdensome ...compositing with metal and ceramic particles, we put forward the idea of approaching impedance match by transforming graphene from diamagnetic to ferromagnetic and meanwhile suppress the conductivity. In this study, we synthesized the Nitrogen-doped graphene (NG) by a facile hydrothermal method with graphene oxide (GO) and urea as precursors. In comparison with GO and reduced GO (rGO), the nitrogen doping along with reduction process boosted the magnetism via a Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism. Pyrrolic-N has been found to dominate the magnetic property induced, which cooperates with the suppression of conductivity to benefit the absorption performance. The reflection loss of nitrogen doped graphene can achieve −11.3 dB absorption maximum at 12.7 GHz and an absorption bandwidth of 12.2–14.3 GHz (reflection loss < −10 dB) at a thickness of 3 mm, which proves to be favourable with respective to the density as compared to existing graphene-based absorbers.
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Most traditional fillers used in electromagnetic shielding composites require high loading, uniform distribution and complicated structures for modest property enhancement. Here, we propose a simple ...shielding system incorporating magnetic microwires M and graphene fibers G in different arrangements. Integrating both fillers in the composite resulted in superior shielding effectiveness (SE) with respect to incorporating either individual type of filler, due to improved absorption efficiency, impedance matching and polarization triggered by different arrangements of the fillers. Randomly dispersed arrays provided a maximum shielding of 6 dB for equal amount of G and M, in which polarization effects at the interface between the two regions contributed to the SE value. The highest SE of 18 dB (98.4% attenuation) was reached by the MMMGGG periodic arrays with merely 0.059 wt% filler loading enabled by the optimized arrangement and wave attenuation from the microwires dielectric/magnetic response. The normalized SE of this composite was about two to four orders of magnitude higher than that of many other shielding candidate materials. High SE, low loading, simple structure, and multifunctionality make these composites attractive for several applications. Moreover, tailoring topological and structural factors of both fillers would facilitate further SE enhancement without sacrificing the important lightweight advantage.
Hybridizing nanocarbons, such as carbon nanotubes (CNT) or graphene, with magnetic metals is a powerful strategy towards designing high-performance microwave absorber due to the resulting synergetic ...loss mechanisms and tunable electromagnetic properties. Herein, CNT and reduced graphene oxide (rGO) have been respectively coupled with amorphous wire through electrodeposition in a complementary fashion. The absorption performance of the hybrid fibers proves to be tunable via controlling the thickness and morphology of the CNT or by regulating the number of oxygen-functional groups of rGO through thermal annealing. The CNT/wire hybrid structure effectively strengthens both the interfacial polarization of CNT through the support of circular-shaped conductive substrate, and the ferromagnetic resonance of wire through the coupling with magnetic impurities in CNT. As for the rGO/wire fiber, dual-band absorption occurs in the sample annealed at 900 °C with an optimal absorption loss of −35 dB at 11.3 GHz, derived from dielectric loss mechanisms such as removal of functional groups and induced defective structure in combination with enhanced magnetic losses in the hybrid structure. Such complementary design opens up new horizons to scale up the excellent assets of nano-carbons into the macroscale and develop functional adaptive materials for high-frequency applications.
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Dual three-phase machines are attractive due to advantages such as inherent fault tolerance. Several strategies for current reference generation have been proposed to improve the postfault ...performance under open-phase fault. However, for the development and analysis of these strategies, only the stator winding losses were considered, but not the converter ones. In fact, there are no studies so far evaluating the converter losses during postfault operation. Aiming to fill this gap, this letter addresses this topic. Namely, it compares the main postfault control strategies in terms of converter losses for dual three-phase machines with sinusoidally distributed windings under single open-phase fault.
•Magnetic properties of Ni2MnGa nanowires are systematically studied for the first time.•Griffith phase theory explains anomalous magnetic behavior disobeying Curie-Weiss law.•Qualitative and ...quantitative methods determine second order magnetic phase transition.•Ni2MnGa nanowires have broad (150 K) and shallow (0.35 J/kgK) magnetic entropy change.•Magnetocaloric response is analyzed by constructing phenomenological universal curves.
Heusler alloys are favorable candidates for fabricating functional devices and sensors due to their characteristic structural and magnetic properties, which vary at different length scales. In this paper, for the sake of expanding the fields of application and systematically studying the mechanisms at the nanoscale, Ni2MnGa Heusler alloy nanowires were fabricated via the electrospinning method followed by optimized heat treatments. While the nanowires exhibited ferromagnetic-paramagnetic transition near room temperature, anomalies in magnetic behaviors were observed by power fitting of χ−1-T curves disobeying Curie-Weiss law. The Ni2MnGa nanowires exhibited maximum isothermal entropy change of 0.35 J/kgK accompanied by extremely wide working temperature region (150 K), with a peak temperature (305 K) suitable for room temperature applications. Arrott plots and quantitative analysis of field/temperature dependence of magnetic entropy change determined second order magnetic phase transition in the nanowires. Construction of phenomenological universal curves addressed demagnetizing effect on magnetocaloric response, which was compared to the multiphase composition obtained by structural analysis. These novel findings in Ni-Mn-Ga nanowires enrich our knowledge of structure and magnetism of Heusler alloys at the nanoscale and could be taken as reference facilitating future relevant research.
Barium titanate (BT) is a well-known electroceramic attractive for microwave applications owing to its exceptional ferroelectric characteristics, elevated dielectric constant, and piezoelectricity. ...BT is usually blended with conductive/magnetic fillers (mostly 0D and 3D micro/nanostructures) to better tailor its electromagnetic properties based on synergistic effects. However, complex synthesis, filler agglomeration, and high ceramic filler concentration inevitably lead to flexibility degradation, limiting its application in modern electronics. Here, we exploit magnetoelectric coupling effects in which ferroelectricity and ferromagnetism coexist to modulate the shielding properties of BT/silicone rubber composites via incorporating ferromagnetic microwires. The high dielectric constant of BT and electric hysteresis effects contributed to microwave attenuation. At the same time, its piezoelectricity modified the wire’s magnetic anisotropy via interfacial strain. In turn, the microwires promoted magnetic losses mainly by ferromagnetic resonance and magnetic hysteresis while also affecting dipole rotation in BT via wire–wire-dipolar magnetic interaction. The efficient synergism between the fillers resulted in enhanced transmission and shielding tunability, reaching a maximum shielding of 16 dB at 9.5 GHz for composites incorporating 30 wt.% BT and six microwires (0.0126 vol%) compared to 2.26 dB for composites with merely BT. Moreover, benefiting from the elastomer matrix and low filler content, these composites have potential in flexible electronics. At the same time, it also constitutes a platform for designing shielding materials based on ferroelectric/ferromagnetic heterostructures.
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