Including Conduction Losses Research Articles

Enhanced electromagnetic wave absorption performance of Ta₄C₃Tx MXenes with optimized interlayer spacing via hydrofluoric acid etching

Ta4C3Tx MXene, a novel two-dimensional material, shows significant application potential for electromagnetic wave (EMW) absorption because of its unique layered structure and high conductivity. In this study, hydrofluoric acid etching was used to obtain Ta4C3Tx MXenes with varying interlayer spaces by adjusting the etching duration, which enhanced the EMW absorption performance. The impact of interlayer spacing on the EMW absorption performance of Ta4C3Tx MXene was systematically evaluated for the first time. The results show that the sample etched with hydrofluoric acid for 48 h (S2-48 h) exhibited the best EMW absorption performance. For S2-48 h, the optimal reflection loss (RL) value was −33.53 dB at a frequency of 9.74 GHz and a thickness of 1.4 mm. In addition, the widest effective absorption bandwidth was 2.97 GHz (10.73–13.7 GHz), which was achieved at a frequency of 11.72 GHz and a thickness of 1.2 mm. This outstanding performance is mainly attributed to good impedance matching and multiple loss mechanisms induced by the etching process, including conduction loss, dipolar polarization, interfacial polarization, and multiple RLs. Furthermore, this research expands the application potential of two-dimensional MXene materials in EMW absorption.

N-doped carbon layer cladded carbon sphere for broadband and boosted microwave attenuation capacity.

Structural construction and heteroatom doping are deemed effective strategies for designing a high-performance microwave absorbing composite to eliminate electromagnetic hazards. Herein, a series of core-shell structural carbon@N-doped carbon (C@NC) nanospheres were successfully fabricated without employing additional modifying agents and sophisticated operation. After incorporating them into a polyvinylidene fluoride (PVDF) matrix, the C@NC/PVDF composites possess tunable wave attenuation capacity obtained by regulating the coating layer thickness and filler loading. Benefitting from the design strategy of the core-shell structure and N-doped C, the C@NC-2/PVDF composites displayed the broadest effective absorption bandwidth of 6.29 GHz (11.71-18 GHz) under a filler content of only 10 wt% at 2.01 mm. Additionally, the minimum reflection loss value of C@NC-3/PVDF composites achieves -62.87 dB within the same mass ratio at a thickness of 2.35 mm. The excellent wave dissipation ability is attributed to the combination of optimized impedance matching and synergistically enhanced multiple loss mechanisms including conduction loss, interfacial polarization as well as dipolar polarization. This work offers a ponderable paradigm for the deeper exploitation of high-efficiency carbon absorbers.

Fabrication of an Ultralight Ni-MOF-rGO Aerogel with Both Dielectric and Magnetic Performances for Enhanced Microwave Absorption: Microspheres with Hollow Structure Grow onto the GO Nanosheets.

An ultralight Ni-MOF-rGO aerogel which possess the merits of not only broad bandwidth and strong absorption but also lightweight and thin matching thickness is fabricated through a hydrothermal treatment, freeze-drying, and annealing procedure. The Ni@C microspheres are dispersed randomly and evenly on the graphene oxide (GO) nanosheets, which can be proved through SEM and TEM results. The electromagnetic parameters of the composite can be adjusted by changing the mass ratio of the MOF and GO to endow the material with both good impedance matching and superior electromagnetic wave absorption performances. Consequently, the resulting composite shows outstanding microwave absorption performance, which achieves strong absorption (-51.19 dB) and broad effective absorption bandwidth (6.32 GHz) with a thickness of 1.9 mm while the filling content is only 2 wt %. In addition, the multiple loss mechanisms of the Ni-MOF-rGO aerogel are illustrated, including conduction loss, dipolar polarization, interfacial polarization, magnetic resonance, and eddy current loss. In a word, the extraordinary microwave absorption performance is ascribed to the synergistic effects of the unique multiple layered structure of GO and the hollow core-shell structure of the Ni@C microsphere. This work demonstrates that the ultralight aerogel with excellent electromagnetic wave absorption performance is a promising strategy for microwave absorption application.

From the solid waste mixture of coal hydrogasification residue and red mud to efficient Fe3O4/C- and Fe/C-based composite microwave absorbents

Following the concept of sustainable development, magnetic particles/C-based composite microwave absorbing materials were prepared through reactions between two solid wastes: coal hydrogasification residue (CHR) and red mud (RM). At a fixed mass ratio of CHR to RM of 0.6:1, Fe3O4/C- and Fe/C-based composites could be obtained at 700 and 900 °C, respectively. A minimum microwave reflection loss of −48.3 dB and a largest effective absorption bandwidth of 4.5 GHz were achieved for the Fe/C-based composite with a simulated thickness of 1.5 mm. The Fe3O4/C-based composite exhibited slightly poorer performance because the fixed initial system could not guarantee impedance matching for both products. The strong intrinsic attenuation capacity of the materials mainly resulted from the dielectric loss, including conduction loss caused by graphite carbon as well as interfacial polarization relaxation loss arising from the numerous phase boundaries.

Facile synthesis of La2O3/Co@N-doped carbon nanotubes via Prussian blue analogues toward strong microwave absorption

Prussian blue, a member of the metal-organic frameworks (MOFs), has attracted great interest in electromagnetic wave absorption due to its unique properties, such as multiple morphologies and tunable structures. In this paper, La2O3/Co@N-doped carbon nanotubes (La2O3/Co@NCNTs) composites were synthesized by thermal annealing La[Co(CN)6]·5H2O Prussian blue analogue (LaCo-PBA) precursors under an Argon (Ar) atmosphere. The studies of electromagnetic wave absorption show that La2O3/Co@NCNTs demonstrated the minimum reflection loss (RL) of −53.40 dB with the effective absorption bandwidth (EAB) of 4.34 GHz at the thickness of 2.45 mm. These excellent microwave absorption properties are attributed to multiple loss mechanisms, including conduction loss, polarization loss, magnetic resonance loss, and eddy current loss. This study offers a new possibility for the design of multifaceted lightweight carbon-based microwave absorbing materials.

Facile fabrication of cobalt/nitrogen-doped graphite composites with high-performance microwave absorption

Magnetic metals are vital microwave-absorbing materials in alleviating electromagnetic (EM) pollution; however, in practical applications they are limited by their high density and relatively narrow absorption band. An effective strategy for addressing this issue is compounding magnetic metals with carbon materials. In this work, we fabricated the carbon/nitrogen-doped graphite (Co/NC) composites using a simple and low-cost method by directly calcining a dry mixture of melamine and Co powders in Ar gas. Part of Co/NC composite was calcined at 800 °C (Co/NC−800). The effects of variations in phase composition and morphology on the EM parameters and microwave-absorbing performance were investigated. Compared to pure Co, Co/NC−800 composite exhibits excellent microwave-absorbing performance such as strong absorption, better EM wave attenuation, and a lighter weight. Particularly, Co/NC−800 composite achieved the minimum reflection loss (RL) of −57.76 dB at 4.88 GHz and an effective absorption bandwidth (EABW, RL ≤ −10 dB) of 14.26 GHz (3.74–18.00 GHz) at a coating thickness d of 1.00–5.00 mm. Further, the maximum EABW of 5.50 GHz was attained at d = 1.86 mm. The optimal impedance matching, multiple scattering and reflections of EM waves on rough surfaces, and strong dielectric loss including conduction loss, interfacial and dipolar polarizations were noted as the main factors for the enhanced EM energy absorbance of Co/NC−800 composite. We believe our work may serve as a guideline for the fabrication of other light carbon-based magnetic metal composites as EM wave absorbers.

Optimization of Extended Phase-Shift Control for Full-Bridge CLLC Resonant Converter With Improved Light-Load Efficiency

The capacitor–inductor–inductor–capacitor (CLLC) resonant converters are drawing more and more attention for their superiority in soft switching, wide output range, and symmetrically bidirectional operation. However, CLLC converter still suffers the problems of unsatisfactory voltage regulation and low efficiency under light-load conditions. Therefore, this article proposes an optimization for extended phase-shift (EPS) control in full-bridge CLLC resonant converter to improve the light-load efficiency. In order to study the relations between phase-shifts of EPS control and converter efficiency, a detailed circuit model is first established to solve the voltage gain, time-domain expressions of main circuit variables, and root mean square (rms) values of resonant currents. Then, a comprehensive loss evaluation is conducted by calculating and analyzing the main power losses, including conduction loss, switching loss, and core losses of magnetic components. Besides, zero voltage switching conditions of primary and rectifier switches are derived to define the range of soft switching. Finally, based on these analyses, the optimal combination of phase-shifts for EPS control is determined to achieve the maximum conversion efficiency of full-bridge CLLC converter at light-load conditions. The validity of the proposed optimized EPS control is verified on a 18∼25 V/400 V, 200 W GaN-based CLLC resonant converter prototype.

Optimal Design of Litz Wire Coils With Sandwich Structure Wirelessly Powering an Artificial Anal Sphincter System.

Transcutaneous energy transfer system (TETS) is widely used to energize implantable biomedical devices. As a key part of the TETS, a pair of applicable coils with low losses, high unloaded Q factor, and strong coupling is required to realize an efficient TETS. This article presents an optimal design methodology of planar litz wire coils sandwiched between two ferrite substrates wirelessly powering a novel mechanical artificial anal sphincter system for treating severe fecal incontinence, with focus on the main parameters of the coils such as the wire diameter, number of turns, geometry, and the properties of the ferrite substrate. The theoretical basis of optimal power transfer efficiency in an inductive link was analyzed. A set of analytical expressions are outlined to calculate the winding resistance of a litz wire coil on ferrite substrate, taking into account eddy-current losses, including conduction losses and induction losses. Expressions that describe the geometrical dimension dependence of self- and mutual inductance are derived. The influence of ferrite substrate relative permeability and dimensions is also considered. We have used this foundation to devise an applicable coil design method that starts with a set of realistic constraints and ends with the optimal coil pair geometries. All theoretical predictions are verified with measurements using different types of fabricated coils. The results indicate that the analysis is useful for optimizing the geometry design of windings and the ferrite substrate in a sandwich structure as part of which, in addition to providing design insight, allows speeding up the system efficiency-optimizing design process.

State-space average-value modelling of capacitor-based switching stages: including conduction losses

State-space average-value models of a second-order pulse-width modulated DC-DC converter have been proposed in the literature to represent the non-ideal inductor-based switching stages. This Letter extends previous works and proposes a new full-order, state-space average-value model of fourth-order DC-DC converters, which considers the non-ideal capacitor-based switching stages. The proposed model is seamlessly functional in all operational modes and has been verified by hardware measurement and detailed simulation in both time and frequency domains.

Frequency-dependent resistance in Litz-wire planar windings for domestic induction heating appliances

In this paper, the frequency-dependent resistance in Litz-wire planar windings for domestic induction heating appliances is analyzed. For these inductors, in which the size is not an essential constraint, an analytical model is developed based on the superposition of different loss effects in the wire. Eddy current losses, including conduction losses and proximity-effect losses, both internal and external, were considered and modeled. The magnetic field necessary to evaluate the external proximity losses is as well analytically calculated considering the complete winding and load properties. To verify this model and its limitations, several inductors with different wires and numbers of turns were constructed and results with both non-loaded and loaded inductors are compared with theoretical predictions.

A new averaged switch model including conduction losses for PWM converters operating in discontinuous inductor current mode

In this paper a new PWM switch model including conduction losses, for Discontinuous Inductor Current Mode (DICM) dc-dc converters is developed. Compared to other models for DICM, the proposed model is more accurate, its derivation considering the correct, exponential waveforms for the currents and voltages. The proposed model is verified through PSpice simulation. A possibility of simplifying the model is also presented and verified through PSpice simulation.

Simple method of including conduction losses for average modelling of switched-inductor cells

Several sophisticated averaged-switch models have been proposed in the literature to represent the non-ideal pulse width modulated switched-inductor DC-DC converters based on the energy conservation principle. Alternatively, proposed is a full-order state-space averaged model that takes into account conduction losses associated with the switching cell's components and is seamlessly functional in all operational modes. The proposed model also demonstrates improvement in accuracy compared to several established models.

Simple resistance calculation in litz-wire planar windings for induction cooking appliances

We present a simple method to calculate the winding resistance of a litz-wire inductor in an induction cooking appliance, taking into account eddy-current losses, including conduction losses and proximity-effect losses. The method, which uses numerical calculation of the magnetic field, can be used to determine the optimum number of strands in the litz wire. The effects of the temperature on the winding total resistance are included in the calculation. We have built a prototype to verify the calculation method and its limitations. We consider the accuracy limits of the method.