Equivalent Circuit Model Analysis Research Articles

Low-profile, wideband RCS-reduction antenna array based on a metal–ink hybrid metasurface

In this study, a highly flexible metal–ink hybrid metasurface that can be integrated with planar structures is proposed. The metasurface achieves wideband absorption performance while maintaining an efficient transmission window. Based on equivalent circuit model analysis, an aperture-coupled antenna array was integrated with the metasurface, enabling simultaneous wideband radar cross-section (RCS) reduction and high-performance radiation. The integration of the metasurface and antenna array allowed the overall structure to maintain a low profile. Simulation results show that the proposed array achieves a 10 dB RCS reduction against a metal plate of the same size over the [2.03, 8.11] GHz range, with a fractional bandwidth of 119.9% under x-polarized wave incidence. Under y-polarized wave incidence, a 10 dB RCS reduction is achieved at both [2.12, 5.1] GHz and [6.1, 8.06] GHz, with fractional bandwidths of 82.5% and 27.6%, respectively. Moreover, the array maintains a peak gain of 26.3 dBi and sidelobe levels less than -18 dB. The experimental results align well with the simulated results. The proposed low-RCS antenna array offers potential advancements in planar antenna technology, enhancing stealth capabilities.

Temperature tunable broadband filter based on hybridized vanadium dioxide (VO2) metasurface

Abstract In this paper, a novel temperature tunable terahertz (THz) broadband filter based on hybridized vanadium dioxide (VO2) metasurface (MS) was proposed. The designed MS is composed of subwavelength metallic square-grid structure situated between two layers of metallic square-patch structure integrated with VO2 film pad spaced with two layers of dielectric substrate. Utilizing the insulator-metal phase transition characterizations of VO2 in the THz regime, the operation mode of the proposed MS filter accomplishes the broadband transmission-to-reflection transition. Simulation results show that when VO2 is in the insulating state with a lower external temperature, the proposed MS behaves like a broadband filter with transparent window and a transmittance of above 80% over the frequency range of 0.66–1.12 THz. However, when VO2 undergoes the metallic state with a higher external temperature, the MS becomes a broadband filter with low-transmission shielding and exhibiting a transmittance of below 10% across the frequency spectrum from 0.56 THz to 1.4 THz. The physical mechanism of the proposed MS based tunable broadband filter is illustrated by introducing impedance matching theory, equivalent circuit model and field analysis. In addition, the proposed MS structure offers exceptional angular stability and polarization insensitivity, opening up new opportunities for the utilization of energy selective surface in THz applications.

Construction and Analysis of Battery Equivalent Circuit Model Considering Liquid Phase Lithium Ion Diffusion Under High-Rate Conditions

In order to obtain the internal state of lithium-ion battery quickly and accurately, the problem that the internal state of the battery is difficult to estimate quickly and accurately is solved. The discharge performance of lithium ion is affected by the electrode current during the charge and discharge process. This paper analyzes the key factors of the electrochemical model at high rate, considers the change of lithium ion concentration in the liquid phase of the battery, integrates the electrochemical mechanism and the equivalent circuit, and establishes a mechanism equivalent circuit model with more computational advantages through circuit elements. The equivalent circuit model can describe the physical characteristics of the lithium battery. Under high rate constant current and dynamic conditions, the model shows good electrical characteristics. Compared with the traditional empirical equivalent circuit model, the accuracy of the model is improved, and the state quantity is reduced by 15 compared with the electrochemical model. The model is of great significance in practical application.

Analysis of electromechanical equivalent circuit model for the functionally graded tubular piezoelectric transducer

Abstract In this paper, a functionally graded tubular piezoelectric transducer (FG-tPET) is proposed. Based on the constitutive equation and wave equation, the electromechanical equivalent circuit model (EECM) of FG-tPET is obtained. When the non-uniformity coefficient tends to zero, the EECM can be simplified as the EECM of the traditional single-tube piezoelectric ceramic transducer. The correctness of the theoretical model is verified in comparison with the experimental results in the literature. Based on the theoretical model, the influence of the geometric size and the non-uniform coefficient of the transducer on the resonance/anti-resonance frequency is analyzed. The results show that the geometric size and the non-uniform coefficient can realize the coordinated adjustment of the resonance/anti-resonance frequency.

Study of Solid-State Diffusion Impedance in Li-Ion Batteries Using Parallel-Diffusion Warburg Model

Anomalous diffusion impedance due to the solid-state Li+ diffusion in Li-ion batteries is often troublesome for the analysis. In this work, we propose a novel analytical Parallel-diffusion Warburg (PDW) model and couple it with the conventional equivalent electrical circuit model (EECM) analysis to tackle this long-standing challenge. The analytical expression of the PDW is derived from the classical Fickian diffusion framework, introducing non-unified diffusion coefficients that originate from the diverse crystalline conditions of Li+ diffusion paths, as theoretically demonstrated in the atomistic modeling results. The proposed approach (EECM + PDW) is successfully employed to study the diffusion impedance of thin-film LiNi0.5Mn1.5O4 (LNMO) electrodes and porous LiNi0.80Co0.15Al0.05O2 (NCA) electrodes, demonstrating the applicability and robustness of this method.

An ultra‐miniaturized single‐layer multiband frequency selective surface with vent for wireless communication

SummaryA novel single‐layer ultra‐miniaturized multi‐band Frequency Selective Surface (FSS) with a circular vent is designed for shielding LTE and ISM bands. The top layer of the proposed FSS unit cell incorporates a circular patch along with convoluted arms. The designed FSS unit cell exhibits band‐rejection characteristics with a resonance of 800 MHz, 2.45 GHz, and 5 GHz and bandwidths of 523 MHz, 687 MHz, and 725 MHz, respectively. The periodicity of an ultra‐miniaturized FSS unit cell is 0.024λօ x 0.024λօ, encompassing a spatial vent hole of 0.01λօ, where λօ corresponds to the lowest operating frequency. Moreover, the proposed FSS unit cell remains unaltered angular stability up to 75° with less than 1% frequency deviation in TE and TM modes. Good polarization insensitivity for the designed four‐fold symmetrical FSS unit cell is achieved. Further, an Equivalent Circuit Model (ECM) analysis is developed to infer the physical insight of the proposed FSS unit cell. The FSS prototype is measured for the assertion, and the good agreement between the findings from simulation and measurement is validated. For real‐time assertion, the FSS prototype is verified using the Amitech SDR04. Thus, FSS is a promising candidate with good ventilation for shielding the LTE, Bluetooth, and Wi‐Fi application bands.

Inverse design of dual-bandpass metasurface filters empowered by the multi-objective genetic algorithm

Multi-band metasurface filters are becoming increasingly pivotal in high-capacity communication technologies. Traditional methods for designing metasurface structures, to date, have relied on empirical approaches to obtain target electromagnetic responses, thereby suffering from low efficiency. Here, we demonstrate an advanced approach for the inverse design of a multi-band metasurface filter, which consists of a multi-objective genetic algorithm (MOGA) in tandem with equivalent circuit model (ECM) analysis. This integration converts specific frequency response requirements into ECM parameter constraints, significantly streamlining the metasurface design and optimization process and offering superior solutions. Using this inverse design method, we theoretically propose a dual-bandpass metasurface filter which can exhibit transmission passbands in any two adjacent frequency ranges among the X-, Ku-, and K-bands. Further, numerical simulations validate the performances of the proposed device, which show great agreement with MOGA-based predictions. Our results pave the way to the effective inverse design of multi-passband metasurface filters which are useful in many applications, such as microwave filters, radar and satellite communication systems, and radio frequency identification devices.

Curved and Annular Diaphragm Coupled Piezoelectric Micromachined Ultrasonic Transducers for High Transmit Biomedical Applications.

In this paper, we present a novel three-dimensional (3D) coupled configuration of piezoelectric micromachined ultrasound transducers (pMUTs) by combing a curved and an annular diaphragm for transmit performance optimization in biomedical applications. An analytical equivalent circuit model (EQC) is developed with varied excitation methods to incorporate the acoustic-structure coupling of the curved and annular diaphragm-coupled pMUTs (CAC-pMUTs). The model-derived results align well with the reference simulated by the finite element method (FEM). Using this EQC model, we optimize the key design parameters of the CAC-pMUTs in order to improve the output sound pressure, including the width of the annular membrane, the thickness of the passive layer, and the phase difference of the driving voltage. In the anti-phase mode, the designed CAC-pMUTs demonstrate a transmit efficiency 285 times higher than that of single annular pMUTs. This substantial improvement underscores the potential of CAC-pMUTs for large array applications.

Conformal ultra‐miniaturized frequency selective surface for medical shielding applications

SummaryThis article presents a novel wearable frequency selective surface (FSS) for shielding the communication link of implantable devices with the external environment. The proposed shield blocks a dual‐band frequency operating at 404 MHz and 2.45 GHz in the medical implant communication service (MICS) and industrial, scientific, and medical (ISM) bands. The two distinct stop bands provide a high‐frequency band ratio of 6.02 in an ultra‐miniaturized physical form factor of 0.024λo × 0.024λo, with at least 76.96% miniaturization when compared to the existing art. Each of the unit cells is imprinted on a conformal 50 μm polyester substrate in a symmetrical configuration. The polyester adheres with jean to realize its compatibility with wearable applications. The designed shield system offers a low profile of only 0.00146λo. By suppressing the grating lobe, the proposed shield achieves a high angular stability of 80° and polarization insensitivity of 90° in both transverse electric (TE) and transverse magnetic (TM) modes. The FSS is fabricated and measured results validate its performance. To evaluate the physical insight, an equivalent circuit model (ECM) analysis is done. To ensure durability, the proposed shield system is tested with Amitech SDR04.

Investigation of lead-acid battery water loss by in-situ electrochemical impedance spectroscopy

Understanding the chemical reactions that occur during lead-acid battery aging is useful for predicting battery life and repairing batteries for reuse. Current research on lead-acid battery degradation primarily focuses on their capacity and lifespan while disregarding the chemical changes that take place during battery aging. Motivated by this, this paper aims to utilize in-situ electrochemical impedance spectroscopy (in-situ EIS) to develop a clear indicator of water loss, which is a key battery aging process and could be repaired, through unique water loss experiments. These experiments were designed to ensure that the percentage of water in the electrolyte or the volume of electrolyte was the only factor that affected in-situ EIS in each experiment, and these values were regularly changed to simulate water loss in a specially designed transparent lead-acid battery. Through an improved equivalent circuit model (ECM) and grey relation analysis (GRA), this work shows that the variation of double-layer capacity and internal resistance can indicate added water content and electrolyte volume. The developed method is simple and can be applied to identify and respond to battery water loss effectively.

A Slot-Connected Cavity Design With Corresponding Equivalent Circuit Model Analysis for Fully Metallic 3-D Vivaldi Antenna for Wireless Power Telemetry Applications

A Slot-Connected Cavity Design With Corresponding Equivalent Circuit Model Analysis for Fully Metallic 3-D Vivaldi Antenna for Wireless Power Telemetry Applications

A single-layer band-stop frequency selective surface with wide angular stability property

AbstractIn this paper, a single-layer band-stop frequency selective surface (FSS) by combination of Mike Kastle unit and square ring unit is proposed to achieve wide angular stable shielding. Owing to the rotational symmetry of the structure, the designed FSS is insensitive to polarization. By the combination design, the wide angular stability can be achieved as the incident angle increases from 0° to 85°, with only a maximum frequency deviation of 0.012 GHz. Meanwhile, the mechanism of the proposed FSS is investigated by the parametric analysis of equivalent circuit model. The prototype was manufactured and measured to verify the design and simulation analysis, and the measurement results were in good agreement with the simulation results.

Impedance Variation of All-Solid-State Battery Cell Using Li10GeP2S12; States-of-Charge Dependence Visualized by Distribution-of-Relaxation Time Analysis

Electrochemical impedance spectroscopy (EIS) will support understanding of electrochemical phenomena in emerging energy devices, including all-solid-state Li ion batteries (ASSLiBs), only when the results are analyzed in a consistent way without arbitrariness. This study1 aimed to establish such a robust analysis for ASSLiB cells using a fast lithium ion conductor Li10 +x Ge1+x P2 −x S12 (LGPS)2, by applying distribution-of-relaxation time (DRT) analysis3. The DRT method distinctively visualized impedance changes depending on states of charge (SOCs). Pellet-type cells were prepared with an In–Li anode, LGPS separator, cathode composite comprising LGPS and LiNbO3-coated LiCoO2 powders4. Figure 1a shows the discharge curve that was recorded for the cell at the current density of 13.2 μA cm−2 (1/40 C-rate) and with the cut-off voltage of 1.9–3.6 V vs. Li+/In–Li (2.55–4.25 V vs. Li+/Li). The EIS measurements were performed during discharge process at various SOCs from 100% to 20%. Nyquist diagrams obtained from the EIS data are shown in Figure 1b, where two broad semicircles are observed at low and high frequency sides. This result indicates at least two electrochemical processes exist in the cell. Both semicircles become large in size upon decreasing SOCs from 100 % down to 20%, indicating that the cell has SOC-dependent electrochemical processes. Figure 1c shows DRT transformation of the EIS data, which helps to separate impedance components in detail. Here horizonal and vertical axes respectively represent the measurement frequency and polarization contribution at each frequency. The center frequency of each distinct peak corresponds to time constant [τ =1/(2πf)] for the respective electrochemical process, and thus the number of peaks matches that of electrochemical processes in the cell. Meanwhile, the peak area represents polarization contribution of each process. Consistently in the whole SOC range, the above DRT analysis indicates the semicircle at high frequency side is divided into two impedance contributions and that at low frequency side cannot be separated any more. This result is consistent with the DRT spectra from additionally prepared symmetric cells; in total three DRT peaks were observed with two peaks from interphase and interface between In–Li/LGPS, and one from LiNbO3-coated LiCoO2/LGPS interface. Accordingly, DRT diagram in Figure 1c was interpreted as indicated by arrows for each peak. This study demonstrated that DRT analysis supports a consistent interpretation of EIS data from ASSLiB cells at various SOCs and with different electrode configurations, and thus implied that the method will also help subsequent equivalent circuit model analysis for reproducible parametrization of electrochemical processes.1. S. Hori et al., “Understanding the impedance spectra of all-solid-state lithium battery cells with sulfide superionic conductors,” J. Power Sources 556, 232450 (2023).2. N. Kamaya et al., “A lithium superionic conductor,” Nat. Mater. 10, 682–686 (2011).3. S. Dierickx, A. Weber, E. Ivers-Tiffée, “How the distribution of relaxation times enhances complex equivalent circuit models for fuel cells,” Electrochim. Acta 355, 136764 (2020).4. N. Ohta et al., “LiNbO3-coated LiCoO2 as cathode material for all solid-state lithium secondary batteries,” Electrochem. Commun. 9, 1486-1490 (2007). Figure 1

An ultra-thin, low-RCS, dual-bandpass novel fractal-FSS for planar/conformal C&X bands applications

In this paper, the authors introduce a novel fractal-frequency selective surface (F-FSS) geometry for dual-bandpass applications. The centriole-symmetric geometry and dual-band characteristics have been achieved by introducing a golden ratio-based fractal geometry. The characteristic mode and equivalent circuit model analysis of the proposed geometry have been performed. The design offers significant autonomy in controlling the two resonant frequencies separately. Additionally, it demonstrates robust stability against variations in bending angle and polarization. To verify the concept, 7 × 7 array with a dimension of 0.51λ0×0.51λ0×0.02λ0 (where the free-space wavelength (λ0) is 39.20mm) is simulated, fabricated, and tested in an anechoic chamber. In measurement, the proposed F-FSS geometry achieves notches at 7.74 and 11.80 GHz, with fractional bandwidth of 6.10% and 3.90%, respectively. The design showcases a thickness-bandwidth ratio (T-BR) of 0.62 and 1.54 within their respective frequency bands. The measured results match closely with the simulation results and yield a low RCS value.

Enhanced stability and electrochemical investigations of Ni/ZSM-5 catalyst layer on nickel-based anodes for ammonia-fed solid oxide fuel cells

Ammonia-fed solid oxide fuel cell (SOFC) is a promising clean energy technique with distinctive advantages of zero-carbon emission, easy storage, and high efficiency. However, the typical nickel anode for SOFC is susceptible to structural change and performance degradation under an ammonia environment, particularly at reduced operating temperatures. In addition, obtaining a deep understanding of the individual electrode processes is demanded for the rational design and optimization of NH3–SOFCs. This work enhances the cell performance and durability of ammonia-fed SOFCs by utilizing a cost-effective ammonia decomposition catalyst (Ni/Na-ZSM). The electrode processes are investigated quantitively using distributed relaxation time (DRT) and equivalent circuit model (ECM) analysis. Accelerated degradation tests are conducted for the NH3-based SOFC cells by thermal cycling at operational temperatures of 700 °C-650 °C. Compared to the conventional cell, the cell with the Ni/Na-ZSM catalyst exhibits improved cell performance and five times less decay. In situ measurements and impedance analysis combined with microstructural characterization reveal that the durability enhancement is probably ascribed to protecting the structure of the three-phase boundaries (TPBs), which maintains a stable resistance of the TPB charge transfer processes during the temperature cycling.

A comparative performance analysis of electrical equivalent circuit models with the hysteresis effect of lithium iron phosphate batteries

ABSTRACT This paper aims to develop an equivalent circuit model (ECM) for Lithium Iron Phosphate (LFP) batteries as they have a flat voltage profile and dominant hysteresis behavior compared to other lithium-ion chemistries. Hence, it is challenging to deduce an accurate model to predict the performance of the battery. The model introduces a hysteresis correction factor (HCF) calculated from the capacity test and the R and C parameters for charging and discharging through pulse current test. The state of charge is calculated through Coulomb counting, and over the entire range, the terminal voltage of the model coincides with the experimental voltage. The estimated maximum deviation in the voltage for the existing 2RC model is 45 mV during discharging with a constant C-rate, and for the proposed model, the maximum deviation in the voltage is found to be 30 and 20 mV during discharging and charging, respectively. Further, to increase the accuracy in the estimated voltage, a 3RC model with HCF is found to be the most suitable for LFP batteries. This model reduces the deviation to 10 and 12 mV during discharging and charging, respectively. The performance of the model in MATLAB Simulink is validated with the experimental results through the Dynamic Stress Test (DST) and the Urban Dynamometer Driving Schedule (UDDS). This model reduces the root mean square error (RMSE) in the estimated voltage by 60% under UDDS compared to other existing models.

A novel dataset sampling method for deep learning-based absorption prediction of FSS absorbers

Combining the deep learning (DL) model to design frequency selective surface (FSS) is a promising research topic, while the DL model usually requires massive datasets. In order to reduce the size of the dataset while maintaining the performance of the DL model used to predict absorptivity of FSS absorbers, a novel dataset sampling method is proposed. Based on the bounded-loss theory, it is proposed that the performance of the DL model is determined by the sampling density and the difference between absorptivity of neighboring samples. This difference is measured by the variations of absorptivity corresponding to these samples. The absorptivity are acquired through the equivalent circuit model (ECM) analysis instead of full-wave simulation to provide immediate results during the sampling period. Numeric validations of the single-layer absorber show that the method requires a much smaller dataset size than the conventional random sampling method with the same Mean Square Error performance of the DL model. The three-layer results show that the method is more accurate in predicting the absorption rate when trained with 20% of the total dataset compared to the random method and the ECM method.

Design and analysis of double-layer parasitic split-ring resonator based on O-shaped engraved microstrip based radiating structure for 5G, WiMAX, WLAN and LTE applications

A novel approach of frequency tunability to cover multiple wireless applications by using four PIN diodes as a switching element is presented. Five switching modes of PIN diodes are analyzed for frequency tunability. The parasitic metamaterial superstrate achieves performance improvement shown in the design. Numerical analysis of metamaterial rings and equivalent circuit models of the SRR and PIN diode are also presented. The structure is analyzed regarding reflection response (S11), Gain, VSWR, directivity and E-field for 2 GHz–6 GHz. Computational analysis is performed by the HFSS simulator and is compared with the fabricated structure (FR-4) for verification. Copper and liquid-based material are used for performance analysis of all the models, and comparative analysis is included. Liquid-based split ring resonator provides a maximum total gain of 5.564 dB, reflection response of −43.36 dB and tunability of 510 MHz. The proposed antenna is used in 5G, WLAN, WiMAX and LTE applications.

The fatigue effects in red emissive CdSe based QLED operated around turn-on voltage.

The operational stability is a current bottleneck facing the quantum dot light-emitting diodes (QLEDs). In particular, the device working around turn-on voltage suffers from unbalanced charge injection and heavy power loss. Here, we investigate the operational stability of red emissive CdSe QLEDs operated at different applied voltages. Compared to the rising luminance at higher voltages, the device luminance quickly decreases when loaded around the turn-on voltage, but recovers after unloading or slight heat treatment, which is termed fatigue effects of operational QLED. The electroluminescence and photoluminescence spectra before and after a period of operation at low voltages show that the abrupt decrease in device luminance derives from the reduction of quantum yield in quantum dots. Combined with transient photoluminescence and electroluminescence measurements, as well as equivalent circuit model analysis, the electron accumulation in quantum dots mainly accounts for the observed fatigue effects of a QLED during the operation around turn-on voltage. The underlying mechanisms at the low-voltage working regime will be very helpful for the industrialization of QLED.

Low-Profile Higher-Order Narrowband Bandpass Miniaturized-Element Frequency-Selective Surface

We present a low-profile miniaturized-element frequency-selective surface (MEFSS) with narrowband, second-order bandpass response. The miniaturized-element unit cell comprises a capacitively loaded parallel LC unit cell resulting in a narrowband bandpass response. A higher-order filtering response is obtained by implementing five metallic layers FSS formed by two layers of parallel LC resonators with an inductive impedance inverter layer in between them. The proposed design offers a very thin profile and an insensitive frequency response to the polarization of the incident electromagnetic wave. A design procedure is developed for the proposed FSS structure based on an equivalent circuit model analysis. Using this model, a prototype of MEFSS with the second-order bandpass response at 3.34 GHz and a fractional bandwidth of 3.7% is designed, realized, and characterized experimentally. The measurement results of the FSS verify the effectiveness of the FSS design procedure and the stability of its frequency response up to 60° oblique incidence angles.