Sawyer-Tower Circuit Research Articles

Boosting piezoelectric properties of PVDF nanofibers via embedded graphene oxide nanosheets

Tremendous research efforts have been directed toward developing polymer-based piezoelectric nanogenerators (PENG) in a promising step to investigate self-charging powered systems (SCPSs) and consequently, support the need for flexible, intelligent, and ultra-compact wearable electronic devices. In our work, electrospun polyvinylidene fluoride (PVDF) nanofiber mats were investigated while graphene oxide (GO) was added with different concentrations (from 0 to 3 wt.%). Sonication treatment was introduced for 5 min to GO nanosheets before combined PVDF solution. A comprehensive study was conducted to examine the GO incremental effect. Microstructural and mechanical properties were examined using a scanning electron microscope (SEM) and a texture analyzer. Moreover, piezoelectric properties were assessed via various tests including impulse response, frequency effect, d33 coefficient, charging and discharging analysis, and sawyer tower circuit. Experimental results indicate that incorporation of GO nanosheets enhances piezoelectric properties for all concentrations, which was linked to the increase in β phase inside the nanofibers, which has a significant potential of enhancing nanogenerator performance. PVDF-GO 1.5 wt.% shows a notably higher enhancing effect where the electroactive β-phase and γ-phase are recorded to be boosted to ~ 68.13%, as well as piezoelectric coefficient (d33 ~ 55.57 pC/N). Furthermore, increasing impact force encouraged the output voltage. Also noted that the delivered open circuit voltage is ~ 3671 V/g and the power density is ~ 150 µw/cm2. It was observed that GO of concentration 1.5 wt.% recorded a conversion efficiency of ~ 74.73%. All results are in line, showing better performance for PVDF-GO 1.5 wt.% for almost all concentrations.

The Domain and Structural Characteristics of Ferroelectric Copolymers Based on Vinylidene Fluoride Copolymer with Tetrafluoroethylene Composition (94/6).

This paper presents data on the macroscopic polarization of copolymer films of vinylidene fluoride with tetrafluoroethylene obtained with a modified apparatus assembled according to the Sawyer-Tower Circuit. The kinetics of the polarization process were analyzed taking into consideration the contributions of both bound and quasi-free (impurity) charges. It was shown that an "abnormal" decrease in conductivity was observed in fields near the coercive fields. This could be associated with the appearance of deep traps of the impurity charge carriers formed by the polar planes of β-phase crystals. The conductivity data obtained from the charge and current responses differed. It was concluded that chain segments contributing to polarization with sufficiently low fields were present in the amorphous phase. A comparison showed that the average size of β-phase crystals (crystals of X-ray diffraction reflection width) was almost one order of magnitude lower than the domain size obtained using piezoresponse force microscopy (PFM). The analysis of the fast-stage dielectric response before and after polarization indicated that as the external polarizing field increased in the ferroelectric polymer chains, conformational transitions occurred according to the T3GT3G- → (-TT-)n и TGTG → (-TT-)n types. This was accompanied by an increase in the effective dipole moment in the amorphous phase chains. The analysis of the IR spectroscopy data obtained in transmission and ATR modes revealed a difference in the conformational states of the chains in the core and surface parts of the film.

Manganese and iron-doped yttrium borate as an excellent multifunctional inorganic material

Manganese and iron-doped π-YBO3 have been synthesized using a modified epoxide-mediated gel method. The PXRD pattern evaluated the formation of the desired phase and the structural changes. EDS spectra determined the elemental analysis of undoped and doped samples. Raman spectra observed the stretching and bending modes of B-O bonds. The direct band gaps for doped samples were 1.47 and 2.07 eV, respectively, lower than the band gap value of 5.81 eV for π-YBO3. The green and blue indigo emission bands were observed in the photoluminescence spectra. Doped samples showed good magnetic properties as they are antiferromagnetic and ferromagnetic at low temperature (T = 5 K) M-H plot and SQUID measurement. An indigenously built Sawyer-Tower circuit is used to measure ferroelectric hysteresis. Photodegradation studies of RhB were conducted under UV-visible irradiation.

Impact of Annealing Temperature on Electrical Properties of Sol-gel Ba0.90Gd0.10TiO3 Thin Films

Ba0.90Gd0.10TiO3 (BGT) thin films have been fabricated in MFM configuration via sol-gel technique at the different annealing temperature. The dielectric parameters of the films are measured using Impedance Analyzer as a function of frequency. It is found that, at frequency 1 kHz, the measured value of e increases from 57 to 264 as the annealing temperature increases from 600 °C to 900 °C, which is correlated to the improved crystallinity and grain size increment. The ferroelectric hysteresis of the films is analyzed using Sawyer-Tower circuit that shows an enhancement for the ferroelectric properties with annealing temperature, which is also confirmed using C-V characteristics. The leakage current of the films is evaluated via Semiconductor Parameter Analyzer (SPA), which shows that at a certain electric field, the leakage current density increases as the annealing temperature increases, that is attributed to the crystallinity and grain size improvement. The conduction mechanism of the films is deeply investigated through different models to find out that the space charge limited conduction (SCLC) mechanism is the controlling conduction process.

Electrode free hysteresis loop measurement of small dimensional ferroelectric materials

As the basic characterization for ferroelectric material, hysteresis loop measurement based on Sawyer-Tower circuit has restrictions of electrodes, parallel plate capacitor structure and complex compensating circuit, which cannot satisfy the demand for characterization of small dimensional ferroelectric materials. In this work, a new measuring method of hysteresis loop is presented based on probes and high resolution semiconductor parameter analyzer. The capacitance of BiFeO3 film between two probes under sweeping bias voltage was measured and transformed into polarization, and the test under light illumination showed similar hysteresis loop. The hysteresis loop of small lead zirconate titanate ceramic particle was also measured. This study gives much freedom to obtain the hysteresis loop between any two points of small dimensional ferroelectric material of any shape, and to get rid of electrodes, and to conduct in situ measurement under external field.

Fluorescent Colloidal Ferroelectric Nanocrystals.

We report the appearance of ferroelectric behavior arising from a room-temperature cation exchange of cadmium-based semiconductor nanoparticles. Fluorescence retention was achieved through protective CdS shelling before cation exchange with tin(IV) by containing defects in the CdS shell rather than the fluorescent CdSe cores. Ferroelectric response, measured using a Sawyer-Tower circuit, was kept constant, while fluorescence retention increases with an increase in the number of CdS monolayers. At 8 monolayers, fluorescence retention reached 99%, allowing for the addition of ferroelectric applications to the already ever-growing list of quantum dot applications.

Pyroelectric power generation in PLZST material by temperature dependent phase transformation

Efforts are being made to increase the efficiency of energy consumption by effectively using the dissipated heat as a source of renewable energy. Herein, we focused on a heat source wherein the temperature varies with time and recovers electrical energy from thermal energy by the pyroelectric effect. Further, we evaluated the net power density of the (Pb, La) (Zr, Sn, Ti)O3 material that is produced by its phase transformation as a result of increase in temperature. It was measured by the Sawyer-Tower circuit that was installed a diode and switch. The results showed that when the low or high temperature side of the measurement environment is in the antiferroelectric phase, the net power density decreased or became negative. Additionally, we estimated the range of temperature wherein a high-power density could be attained by examining the changes in it due to phase transformation.

MAGNETOELECTRIC COUPLING MEASUREMENT TECHNIQUES IN MULTIFERROIC MATERIALS

Magnetoelectric multiferroics are solid-state materials which exhibit a coupling between ferroelectric and magnetic orders. This phenomenon is known as the magnetoelectric (ME) effect. Multiferroic materials possess a wide range of potential applications in such fields as metrology, electronics, energy harvesting & conversion, and medicine. Multiferroic research is facing two main challenges. Firstly, scientists are continuously trying to obtain a material with sufficiently strong, room-temperature ME coupling that would enable its commercial application. Secondly, the measurement techniques used in multiferroic research are often problematic to implement in a laboratory setting and fail to yield reproducible results. The aim of the present work is to discuss three most commonly used methods in multiferroic studies; the lock-in technique, the Sawyer-Tower (S-T) circuit and dielectric constant measurements. The paper opens with a general description of multiferroics which is followed by mathematical representation of the ME effect. The main body deals with the description of the aforementioned measurement techniques. The article closes with a conclusion and outlook for future research.

Influence of BaO Doping on the Structural, Ac Conductivity, and Dielectric Properties of BiFeO3 Multiferroic Nanoparticles

The Bi1−xBaxFeO3 (BiBaFeO3) multiferroic nanoparticles with different Ba molar concentrations were fabricated in reliance on the solid-state reaction technique. Nanostructures of the prepared samples were confirmed by X-ray diffraction (XRD) together with Fourier transforms infrared (FTIR) spectroscopy, whereas the ac conductivity, dielectric and ferroelectric features were examined depending on the RLC Bridge, and Sawyer–Tower circuit. XRD patterns displayed the creation of rhombohedral–hexagonal single-phase of BiBaFeO3. The formation of BiBaFeO3 multiferroic nanoparticles was confirmed by FTIR spectra. Curie temperature (TC) was observed around 1121–1189 K. Ferroelectric polarization was enhanced with remnant polarization of 88.8 μC cm−2 by Ba2+ ions substitution at x = 0.15 mol%. Besides, ac electrical conductivities as a function of frequency as well as temperature were reported for all BiBaFeO3 multiferroic nanoparticles, which exhibit a strong frequency dependence with conduction mechanism is the correlated barrier hopping (CBH) model. The obtained high polarization and Curie temperature enhance their use in information storage devices.

Correlation of Ba:Gd ratio and film thickness to the dielectric, ferroelectric and leakage current mechanism of nanostructure Ba1-xGdxTiO3 thin films

Perovskite Ba1-xGdxTiO3 thin films at different Ba:Gd ratios (0 = x ≤ 0.3, with 0.05 step) and film thicknesses have been fabricated in MFM configuration using sol-gel technique. The dielectric constant has been measured as a function of frequency in the range of 100 Hz and 1 MHz using impedance analyzer. The results show that at 1 kHz, ε decreases as Gd3+ ratio increases, which attributed to the reduction of tetragonality c/a ratio. Whereas, it increases as film thickness increases, which attributed to grain size increment. The ferroelectric hysteresis of the films is characterized using Sawyer-Tower circuit that shows a degradation for ferroelectric hysteresis as Gd+3 ratio increases due to permanent dipoles and grain size effect. Whereas, it enhances with film thickness due to grains enlargement. The leakage current has been analyzed via semiconductor parameter analyzer in the range from 0 to 8 V. It is found that at distinct electric field, the leakage current density reduces as Gd+3 ratio increases, resultant from grain size reduction. Whereas, it reduces as the film thickness increases according to inverse relation with film thickness. Moreover, SCLC, Schottky and Poole-Frenkel emission mechanisms have been modeled for the films to conclude that the former is the predominant mechanism.

In-plane characterization of PZT thin films for the creation of a general impedance model

The in-plane dielectric and ferroelectric properties of preferentially oriented lead zirconate titanate (PZT) thin films are characterized using interdigitated transducers (IDTs). By combining finite element method simulations and capacitance measurements, values of the dielectric constant of films with thicknesses between 150 nm and 800 nm are obtained. A modified Sawyer–Tower circuit is used to investigate the polarization loops measured in-plane using IDT electrodes. A well-defined hysteresis loop is obtained demonstrating the switching of the polarization of the ferroelectric domains. Leakage current measurements reveal high resistivity and are an indication of the high quality of the PZT film. The obtained characteristics are used to determine the total impedance of the IDT-PZT structure. Here, the structure is represented by an equivalent ladder circuit using the inductance and resistance of the IDT electrodes and the capacitance and conductance of the PZT film. The obtained total impedance matches low frequency measurements.

Origins of Soft-SwitchingCossLosses in SiC Power MOSFETs and Diodes for Resonant Converter Applications

The recent development and commercialization of wide bandgap (WBG) power semiconductors, specifically gallium nitride (GaN) and silicon carbide (SiC), have driven the increase in switching frequency for soft-switching power converters, such as the Class E, Class Φ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , and Class DE resonant inverters and rectifiers. However, prior literature has characterized numerous commercial GaN and SiC devices using the Sawyer-Tower circuit and discovered significant large-signal C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> oss</sub> charge-voltage hysteresis. This C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> oss</sub> hysteresis, equivalent to OFF-state energy loss, is highly dependent on the frequency and voltage across the device, hindering the efficiency and performance of MHz-range soft-switched converters. This article is the first to explain the origin of the C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> oss</sub> loss in SiC power devices as charging and discharging conduction losses at the termination of the device. The loss characteristics relative to the operating voltage, frequency, dV/dt, and temperature are dictated by incomplete ionization. Incomplete ionization also highlights a significant inconsistency between the large-signal C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> oss</sub> behavior and small-signal behaviors, which is often the model used in manufacturers' datasheets and SPICE simulations. The large-signal charge-voltage behavior is transient, where the charge in C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> oss</sub> depends on the rate of the voltage swing across the device. We validate these hypotheses through mixed-mode simulations using the Sentaurus technology computer-aided design (TCAD) and experimentally using commercial and custom SiC devices.

Novel iGSE-C Loss Modeling of X7R Ceramic Capacitors

Due to the large relative permittivity of Class II dielectrics, ceramic capacitors (CCs) from these materials promise significant volume and weight reductions in inverter and rectifier sine-wave filters, and are especially attractive in mobile applications that demand ultrahigh power density. While previous literature found large low-frequency losses in these components, no extensible loss model was proposed to accurately characterize these ferroelectric losses. In this article, we take advantage of prior art on ferromagnetic components in power electronics to propose a Steinmetz parameter-based loss modeling approach for X7R CCs, named the Improved Generalized Steinmetz Equation for CCs, or iGSE-C. This model is verified using the Sawyer-Tower circuit to measure losses in a commercially available X7R capacitor across excitation magnitude, dc bias, temperature, excitation frequency, and harmonic injection. Losses are shown to scale according to a power law with charge, with the resulting Steinmetz coefficients valid across dc bias and slightly varying as the temperature is increased. The iGSE-C accurately predicts losses for typical nonsinusoidal phase voltage waveforms with an error under 8%. Finally, the loss modeling technique is demonstrated for the sine-wave output filter of a bridge-leg arrangement with both low- and high-frequency excitations, with total capacitor losses predicted within 12% accuracy.

Analysis of Large-Signal Output Capacitance of Transistors Using Sawyer–Tower Circuit

A detailed analysis of the Sawyer–Tower method used in the measurement of large-signal output capacitance ( $\boldsymbol {C}_{{\textrm {o}}}$ ) of power transistors is presented, followed by important design recommendations to obtain accurate results. Key factors affecting the proper implementation of the technique, such as power amplifier characteristics, load slew rate, reference capacitor ( $\boldsymbol {C}_{\text {ref}}$ ), and reverse conduction of the device, are addressed, with accompanying simulation and experimental results for Si, SiC, and GaN devices. A thorough investigation of the selection of $\boldsymbol {C}_{\text {ref}}$ is presented, with a new equation to correctly determine its value for a given voltage swing and output capacitance range of the device under test (DUT). We report that the Sawyer–Tower circuit imposes the DUT to enter steady-state reverse conduction under certain conditions, thus leading to charge–voltage ( QV ) hysteresis patterns unrelated to $\boldsymbol {C}_{{\textrm {o}}}$ . Our analysis reveals that the origin of this phenomenon is related to DUT’s leakage current and it could be minimized by the proper selection of excitation frequency. This article intends to provide an effective guide on designing and using the Sawyer–Tower circuit, and to induce further scientific insight on characterizing $\boldsymbol {C}_{{\textrm {o}}}$ .

Direct observation of surface polarization at hybrid perovskite/Au interfaces by dark transient experiments

A distinctive feature of hybrid perovskite light-absorbing materials is the non-negligible ionic conductivity influencing photovoltaic performance and stability. Moving ions or vacancies can naturally accumulate at the outer interfaces (electrode polarization) upon biasing. Contrary to that approach, a modulation of conductive or recombination properties could manifest as an alteration in the low-frequency part of the impedance response, either producing inductive or large capacitive features. Under this last view, capacitances are not the response of polarized structures or charging mechanisms, but result from the modulation of currents. This work intends to provide pieces of evidence that assist us in distinguishing between these two dissimilar mechanisms, namely, real charge polarization and delayed current effects under bias in the dark. The analysis relays upon an experimental technique based on transient charging signals using the Sawyer-Tower circuit. Instead of applying an alternating small perturbation over a steady-state voltage (differential capacitance method), transient charging measures the resulting polarization upon a large bias step under the suppression of dc currents. Our findings reveal that real steady-state charge is indeed induced by the applied voltage in the dark, easily interpreted by means of charged real capacitors with values much larger than the geometrical capacitance of the film. The connection between that polarization and the charging of perovskite/contact interfaces is highlighted.

A novel ferroelectric based on quinuclidine derivatives

The compound [(CH3)2CH-C3H17N][CoBr4] (1) based on quinuclidine derivatives was achieved by the solution synthetic method and characterized by elemental analysis, infrared spectroscopy, single-crystal X-ray structural analysis and dielectric measurement, respectively. Variable-temperature single-crystal X-ray diffraction suggested that the compound underwent the phase transition from the space group C2/c to Cc. The polarization curve was measured using the Sawyer-Tower circuit. The structural phase transitions of 1 was ascribed to the distortion of a [(CH3)2CH-C3H17N]2+ cation from this inorganic–organic hybrid material [(CH3)2CH-C3H17N][CoBr4]. The strong change in dielectric anomalies makes compound 1 a suitable candidate for promising switchable dielectric materials. This work represents a feasible strategy thought for the targeted harvesting of low temperature ferroelectrics.

Output Capacitance Loss Characterization of Silicon Carbide Schottky Diodes

In high-frequency (HF) and very-high-frequency (VHF) rectifiers, silicon carbide (SiC) Schottky diodes exhibit higher losses compared to what is reported in manufacturer-provided simulation models, with additional power loss stemming from energy dissipation during the charging and discharging of the junction output capacitance ( $C_{J}$ ). Because these losses are not included in manufacturer simulation models and have not been well-studied for commercially available SiC Schottky diodes, we have experimentally measured them using the Sawyer–Tower circuit. From these measurements, we compare the losses across manufacturers, current rating, generation, voltage rating, and packaging. We then demonstrate the performance of these devices on a 20-MHz class-DE rectifier and compare their power dissipation from the simulation and experimental measurements. Finally, by incorporating these losses in device power dissipation calculations in various rectifier topologies, we propose suggestions on device selection to optimize converter efficiencies.

Modulating Ferroelectric Response in Colloidal Semiconductor Nanocrystals through Cation Exchange

We report the appearance of ferroelectric behavior in CdSSe nanocrystals arising from a room-temperature cation exchange of semiconductor nanoparticles. Cation exchange reactions were performed on colloidal CdSSe nanocrystals using antimony, tin, and gold salts. The degree of exchange was determined using scanning transmission electron microscopy–energy-dispersive spectroscopy and the ferroelectric response was measured using a Sawyer-Tower circuit. As the percent exchange of tin(IV) was varied, the ferroelectric properties were optimized at ∼30 and 70% as a result of intracrystal atomic displacements. The amount of disorder can be minimized by selecting the exchange cation that exhibits the largest change in saturation polarization with the least amount of exchange. The ability to tune the structure and ferroelectric response of nanostructures could offer new routes toward the development of ferroelectric random-access memory devices as well as lower the cost of energy-harvesting applications.

Energy storage behaviors in ferroelectric capacitors fabricated with sub-50 nm poly(vinylidene fluoride) Langmuir–Blodgett nanofilms

High-energy storage in polymer dielectrics is limited by two decisive factors: low-electric breakdown strength and high hysteresis under high fields. Poly(vinylidene fluoride) (PVDF), as a well-known ferroelectric polymer having a high-breakdown strength (700 MV/m) and a high dielectric constant, is suitable for use as a dielectric capacitor film, but ferroelectric hysteresis from the crystalline phase has prevented its practical application. In our previous study, the ferroelectric switching of crystalline PVDF is suppressed effectively in PVDF-based Langmuir–Blodgett (LB) nanofilms because of its large interfacial effect, even in an extremely high electric field. This study investigated the ferroelectricity and energy storage behaviors of PVDF LB nanofilms at sub-50 nm thicknesses. The ferroelectric hysteresis loops were measured using a Sawyer–Tower circuit in different electric fields. An energy density of 6.0 J/cm3 at 500 MV/m was demonstrated for the 12-nm-thick PVDF LB nanofilm device. Polar polymers with permanent dipoles such as poly(vinylidene fluoride) (PVDF) are suitable for use as high-energy storage density dielectrics because of their high permittivity. This study investigated the ferroelectricity and energy storage behaviors of PVDF Langmuir–Blodgett (LB) nanofilms at sub-50 nm thicknesses. The ferroelectric hysteresis loops were measured using a Sawyer–Tower circuit in different electric fields. An energy density of 6.0 J/cm3 at 500 MV/m was demonstrated for the 12-nm-thick PVDF LB nanofilm device.

Effect of Nb, Ta and Sb Addition on Structure and Electrical Properties of PZT Ceramics

Abstract In this study, the effect of Nb, Ta and Sb addition on the structure, dielectric and ferroelectric properties of the Pb(Zr0.5Ti0.5)O3 ceramics in the morphotropic phase boundary (MPB) region were investigated. Powders of pure PZT, PZT + 0.1 mol% Nb, PZT + 0.1mol% Ta and PZT + 0.1mol% Sb were synthesized by conventional mixed oxide method via a vibro-milling technique. All compositions of powders were uniaxially pressed in pellets and sintered at the temperature of 1275°C for 2h. Phase formation, dielectric and ferroelectric properties of ceramics were characterized by XRD, LCR meter and modified Sawyer–Tower Circuit respectively.