In this letter, a compact broadband high-efficiency RF rectifier is presented for ambient RF energy harvesting (EH). A novel impedance matching network with an additional quarter-wavelength ...short-circuited stub was designed to achieve broadband impedance matching. After the branch was added, the parallel resonance point on track <inline-formula> <tex-math notation="LaTeX">S_{11} </tex-math></inline-formula> did not move, and the remaining points were compressed. The measurement results show that the proposed topology could realize a broadband high-efficiency rectifier for ambient RF EH, and the measured results were basically consistent with the simulation results. The measured efficiency is as high as 71.5% for an input power level of 8 dBm. A 1-2.4 GHz wide frequency band with an efficiency above 50% is achieved for an input power level of 5 dBm with a terminal load of 1.6 <inline-formula> <tex-math notation="LaTeX">\text{k}\Omega </tex-math></inline-formula>. Moreover, the circuit had wide dynamic input power characteristics, whose measured values remained above 50% with an input power of 3-14 dBm.
Efficient reduction of graphene oxide to obtain high-quality graphene nanosheets is desirable for energy storage, catalysis, electronics and environmental remediation. In this brief review, we mainly ...focus on the microwave-assisted production of reduced graphene oxide in three categories: (1) microwave-assisted chemical reduction of graphene oxide; (2) microwave-assisted thermal reduction of graphene oxide; (3) microwave-assisted simultaneous thermal exfoliation & thermal reduction of graphite oxide. We also summarize common techniques for characterizing reduction efficiency and quality of as-obtained rGO.
This letter describes two methods for improving the transmission-conversion efficiency in microwave power transmission (MPT). On one hand, according to the obtained formula, the ...transmission-conversion efficiency can be improved by optimizing the amplitude distribution and phase distribution on the aperture of the transmitting antenna. On the other hand, when the distribution of the aperture is fixed and the equal amplitude with in-phase distribution is taken as an example, the transmission-conversion efficiency can also be improved by adopting nonuniform rectenna array. The transmission-conversion efficiency formula is rederived for this case since the size of partial units increases. Lastly, an MPT system is established, and the corresponding transmission-conversion efficiencies are calculated. Compared with the initial case where the transmitting antenna has an equal amplitude with in-phase aperture distribution and the receiving aperture is composed of a uniform rectenna array, the obtained transmission-conversion efficiencies of optimizing aperture distribution and nonuniform rectenna array increased by 25% and 24.9%, respectively. However, if the transmitting antenna is far from the rectenna array, the transmission-conversion efficiencies in these three cases tend to be the same.
Microwave-assisted chemical reactions have been widely used, but the overheating effect limits further applications. The aim of this paper is to investigate the coupling degree of the electromagnetic ...field and thermal field in microwave-heating chemical reactions whose polarization changes as the reactions proceed. First, the entropy-balance equation of microwave-heating polar-molecule reactions is obtained. Then, the coupling degree of the electromagnetic field and the thermal field in microwave-heating polar-molecule reactions is derived, according to the entropy-balance equation. Finally, the effects of reaction processes on the degree of coupling are discussed. When the time scale of the component-concentration variation is much greater than the wave period during the chemical processes, the degree of coupling is sufficiently small, and the electric and thermal fields are considered as weakly coupled. On the other hand, the degree of coupling may change during the reactions. If the absolute value of the coupling degree becomes larger, due to the change in component concentration, this will lead to a transition from weak coupling to strong coupling.
A wideband superdirective array, composed of a two-element circular monopole configuration, is introduced. The monopoles are placed in close proximity, facing each other on a metal ground. To ensure ...good matching at high frequencies, two pairs of elliptical patches are added to the sides of the monopoles, enhancing the surface current of the circular patch for wideband performance. To achieve equal amplitude excitation and the desired phase difference, a wideband power divider with a phase shifter is designed to feed the antenna array. Simulation and measurement results demonstrate that the proposed wideband antenna array, operating within the frequency range of 2.94–7.93 GHz, exhibits a maximum directivity of 8.36–10 dBi, with an antenna efficiency ranging from 47.86 to 83.18% across the bandwidth. The proposed array has the advantages of miniaturization, high directivity and wideband operation and can be widely used in various portable wireless communication systems, including WLAN (5.05–5.9 GHz), ISM (5.725–5.875 GHz), 5G communication (3.3–3.8 GHz), etc.
In this paper, an oblique aperture ridge waveguide operating at 2450 MHz is proposed, and, using the ridge waveguide, a permittivity measurement system is constructed which can measure the ...permittivity of materials during microwave heating. The system calculates the amplitudes of the scattering parameters by using the forward, reflected and transmitted powers of the power meters, and it reconstructs the permittivity of the material by combining the scattering parameters with an artificial neural network. The system is used to measure the complex permittivity of mixed solutions of methanol and ethanol with different ratios at room temperature, and the permittivity of methanol and ethanol with increasing temperature, from room temperature to 50 °C. The measured results are in good agreement with the reference data. The system allows simultaneous measurement of the permittivity with microwave heating and provides real-time, rapid changes in the permittivity during heating, avoiding thermal runaway and providing a reference for applications of microwave energy in the chemical industry.
A low-profile and lightweight circularly polarized (CP) rectenna array based on coplanar waveguide (CPW) is proposed in this letter. The CPW is adopted for enhanced bandwidths. A CPW rectifier, which ...consists of a matching network, a series diode, and a compact dc-pass filter, is directly integrated on the ground plane of the receiving antenna. The proposed rectenna is fabricated and measured. It features stable conversion efficiencies on three azimuthal planes. The maximum efficiency of 71.2% has been measured with a 400 Ω load at 5.8 GHz. A 4 × 4 rectenna array is also built for a large physical aperture and increased output power. The maximum conversion efficiency of 65.24% is measured with the array. Taking advantage of the immunity to misalignment of the CP receiving antenna, the proposed rectenna and array are suitable to be used in mobile applications for wireless power transmission.
Microwave power combining based on dual 15-kW continuous-wave (CW) magnetrons in S-band is presented. The injection locking on 15-kW CW magnetrons has been successfully accomplished. The ...phase-locking bandwidth of each magnetron reaches 5 MHz with an improved dc power supply. Meanwhile, phase jitters are measured and analyzed briefly. The injection-locked 15-kW magnetrons have stable magnitude and tunable locked phase, which are suitable for coherent power combining. A dual-magnetron microwave power combining system has been designed and developed with an automatic phase controlling mechanism. The microwave power combining efficiency is higher than 95%. It is the first time to demonstrate a successful microwave power combining of dual 15-kW S-band CW magnetrons. The system will find industrial applications in producing diamond film from microwave plasma chemical vapor deposition. Large-power and high-efficiency microwave sources based on power combining from magnetrons are good candidates for industrial applications.
•Patients with KOA have altered static and dynamic functional network connectivity.•KOA have abnormal pain-related information processing of the default mode network, sensorimotor network, cognitive ...control network.•Although abnormalities in dFNCs of KOA patients have been found using the common window size, but the results were not robust.
This study aimed to investigate altered static and dynamic functional network connectivity (FNC) and its correlation with clinical symptoms in patients with knee osteoarthritis (KOA). One hundred and fifty-nine patients with KOA and 73 age- and gender-matched healthy subjects (HS) underwent resting-state functional magnetic resonance imaging (rs-fMRI) and clinical evaluations. Group independent component analysis (GICA) was applied, and seven resting-state networks were identified. Patients with KOA had decreased static FNC within the default mode network (DM), visual network (VS), and cerebellar network (CB) and increased static FNC between the subcortical network (SC) and VS (p < 0.05, FDR corrected). Four reoccurring FNC states were identified using k-means clustering analysis. Although abnormalities in dynamic FNCs of KOA patients have been found using the common window size (22 TR, 44 s), but the results of the clustering analysis were inconsistent when using different window sizes, suggesting dynamic FNCs might be an unstable method to compare brain function between KOA patients and HS. These recent findings illustrate that patients with KOA have a wide range of abnormalities in the static and dynamic FNCs, which provided a reference for the identification of potential central nervous therapeutic targets for KOA treatment and might shed light on the other musculoskeletal pain neuroimaging studies.
Ultrafine NiCo2O4 nanoparticles (approximately 6–10 nm in size) dispersed uniformly along the walls of CNTs were obtained by microwave heating. The composites material exhibited good electrochemical ...performance with encouraging specific capacities (1025 mAh g−1 and 792 mAh g−1 at 100 mA g−1 and 1000 mA g−1 respectively) and capacity retention after 100 cycles (83% at 200 mA g−1).
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•NiCo2O4@CNTs composites were synthesized via a microwave-assisted method.•Two morphologies of NCO were obtained in the system with or without CNTs.•The as-synthesized NiCo2O4@CNTs presented 1D configuration of CNTs.•The composites exhibited more excellent performance than that of counterparts.
The paper reports a microwave assisted rapid synthesis of NiCo2O4@CNTs composite for the first time. Ultrafine NiCo2O4 nanoparticles approximately 6–10 nm in size dispersed uniformly along the CNTs walls are obtained. Interestingly, nanosheets structure is formed in the absence of CNTs. The distinctive morphology difference is attributed to the different nucleating and growing mechanisms. Due to the small NiCo2O4 nanoparticle size derived from microwave enabled fast synthesis and the unique 1D nanostructure, the NiCo2O4@CNTs composites materials exhibit more excellent lithium storage performance than that of counterparts.
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