Sawyer-Tower Method Research Articles

“ μ s P-E curve” and reverse phase transition of antiferroelectric ceramics during ultra-fast charge–discharge

Antiferroelectric materials hold great potential for energy storage applications. However, a significant challenge lies in the disparity ΔW between the rapid discharge energy density Wdis and the recoverable energy density Wre. Quantitative analysis is still lacking, and the ultra-fast reverse ferroelectric–antiferroelectric (FE–AFE) transition behavior at the microsecond scale remains unknown. In this study, a pulse technique was employed instead of the Sawyer–Tower method to obtain the “μs P-E loop” during rapid charge–discharge processes. The “μs P-E curve” clearly illustrates the distinct FE–AFE transition behavior during rapid discharge in comparison to low-frequency conditions. Under pulsed conditions, the FE–AFE transition field was observed to decrease, and even a “remanent polarization” was observed, leading to a reduction in discharge energy during fast discharge. Moreover, through the enhancement of relaxor behavior and the increased diffuseness of FE–AFE switching, the μs P-E loop tended to resemble that observed at low frequencies, thereby resulting in more efficient discharge performance. This study introduced a technique for investigating the ultra-fast FE–AFE transition. Furthermore, it unveiled the origin of ΔW and provided an effective method for achieving high discharge energy density.

Lead-Free Hybrid Organic-Inorganic Ferroelectric: (3,3-Difluoropyrrolidinium)2ZnCl4·H2O.

Hybrid organic-inorganic ferroelectrics (HOIFs) have a wide range of applications in the optoelectronic field in terms of rich optoelectronic properties. Particularly, lead-free HOIFs have attracted extensive attention due to their environmental friendliness, low heavy metal toxicity, and low synthesis cost. However, there are few reports about Zn-based HOIFs due to their uncontrollable ferroelectric synthesis and other reasons. Here, we designed and synthesized a zinc-based zero-dimensional (3,3-difluoropyrrolidine)2ZnCl4·H2O (DFZC) single crystal, which undergoes a phase transition from ferroelectric to paraelectric phase (space group from Pna21 to Pnma) at 295.5 K/288.9 K during the heating/cooling process. The systematic study shows that the ferroelectric phase transition is a displacive type. The ferroelectric hysteresis loop of DFZC was obtained by the double-wave method and the Sawyer-Tower method, which has a spontaneous polarization (Ps) of ∼0.4 μC/cm2. This work reveals the strategy to design new zinc-based lead-free HOIFs for potential applications in optoelectronic fields.

A room temperature ferroelectric material with photoluminescence: (1,3-dicyclohexylimidazole)2MnCl4.

Molecular ferroelectric materials have been widely used in capacitors and sensors due to their low cost, light weight, flexibility and good biocompatibility. Organic-inorganic hybrid complexes, on the other hand, have received a great deal of attention in the luminescence field due to their low cost and simple preparation. The combination of ferroelectricity and photoluminescence in organic-inorganic hybrid materials not only leads to tunable optical properties, but also enriches potential applications of multifunctional ferroelectrics in optoelectronic devices. Here, we report a new luminescent ferroelectric material (1,3-dicyclohexylimidazole)2MnCl4 (DHIMC). Thermogravimetric analysis (TGA) was used to measure the mass change of the material at a measurement rate of 20 K min-1 from room temperature to 900 K, and we found that this material has good thermostability, which is up to 383 K. Meanwhile, UV-vis measurements showed that it is also a fluorescent material emitting a strong green fluorescence at the wavelength of 525 nm. The ferroelectricity of the crystal was determined by two different methods: the Sawyer-Tower method and the double-wave method (DWM). Particularly, the single crystal experiences a phase transition from the ferroelectric phase to the paraelectric phase during the heating/cooling process at 318 K/313 K and the space group changes from P1̄ (centrosymmetric) to P1 (non-centrosymmetric). This work will enrich multifunctional luminescent ferroelectric materials and their application in display and sensing.

Oxygen-vacancy induced ferroelectricity in nitrogen-doped nickel oxide

This paper reports the onset of ferroelectricity in NiO by breaking the crystallographic symmetry with oxygen vacancies created by N doping. Nitrogen-doped NiO was grown at room temperature by RF sputtering of Ni target in Ar–O2–N2 plasma on silicon and fused silica substrates. The impact of the nitrogen doping of NiO on microstructural, optical, and electrical properties has been investigated. According to x-ray diffraction investigations, by increasing the N doping level in NiO, a transition from (002) to a (111) preferential orientation for the cubic NiO phase was observed, as well as a lattice strain relaxation, that is usually ascribed to structural defect formation in crystal. The x-ray diffraction pole figures the presence of a distorted cubic structure in NiO and supports the Rietveld refinement findings related to the strain, which pointed out that nitrogen doping fosters lattice imperfections formation. These findings were found to be in agreement with our far-infrared measurements that revealed that upon nitrogen doping a structural distortion of the NiO cubic phase appears. X-ray photoemission spectroscopy measurements reveal the presence of oxygen vacancies in the NiO film following nitrogen doping. Evidence of ferroelectricity in nitrogen-doped NiO thin films has been provided by using the well-established Sawyer–Tower method. The results reported here provide the first insights on oxygen-vacancy induced ferroelectricity in nitrogen-doped nickel oxide thin films.

Temperature-dependency of ferroelectric behavior in CH3NH3PbI3 perovskite films measured by the Sawyer–Tower method

Temperature-dependency of ferroelectric behavior in CH3NH3PbI3 perovskite films measured by the Sawyer–Tower method

Ferroelectric Liquid-Crystalline Binary Mixtures Based on Achiral and Chiral Trifluoromethylphenylterthiophenes

This paper presents a new family of ferroelectric smectic liquid-crystalline binary mixtures composed of achiral and chiral trifluoromethylphenylterthiophenes. The chiral symmetry breaking of the ferroelectric smectic phases can lead to chiral photovoltaic (CPV) effects, as a type of ferroelectric photovoltaic (FePV) effect, which is caused by the internal electric field originating from the spontaneous polarization. These ferroelectric properties were examined using the Sawyer-Tower method, and the CPV effect was confirmed by measuring the steady-state photocurrent response under zero bias. We found that the remnant polarization and photocurrent density in the polarized ferroelectric phases increased nonlinearly with the increase in the content of the chiral component in the mixture. Moreover, the hole mobility evaluated by time-of-flight measurements was kept constant by varying the composition. More than 40 mol % of the chiral component was required to form the polar structure, inducing the CPV effect. Binary mixture systems are advantageous for not only optimizing liquid crystal structures and temperature ranges but also facilitating the design of materials exhibiting CPV effects.

Enhancement of Spontaneous Polarization and Acid Vapor-Induced Polymerization in the Thin-Film States of Phenylterthiophene Derivative Bearing a Cyclotetrasiloxane Ring

We have achieved the measurement of spontaneous polarization of spin-coated thin films of a ferroelectric liquid crystal based on a phenylterthiophene skeleton. Uniaxially aligned spin-coated films with a thickness of 100 nm were prepared by the friction transfer method. The spontaneous polarization of the spin-coated films was measured by the Sawyer–Tower method and polarization inversion current technique under the application of triangular-wave bias. These measurements revealed the spontaneous polarization of 50 nC cm−2 in the thin-film state which is 1.5 times larger than the value of 35 nC cm−2 in the bulk state. This result should be attributed to the strong anchoring of the liquid-crystalline molecules on the substrate surface and the inhibition of polarization relaxation caused by the formation of the helical structure. Moreover, insolubilization of the thin films was carried out by an in situ ring-opening polymerization induced by acid vapors. The molecular alignment in the thin films of the ferroelectric liquid crystal was retained during the polymerization process.

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 $\boldsymbol {C}_{{\textrm {o}}}$ .

Measurement of Large-Signal C OSS and C OSS Losses of Transistors Based on Nonlinear Resonance

In this letter, we present a new measurement technique to evaluate the large-signal output capacitance ( C OSS) of transistors as well as the C OSS energy dissipation ( E DISS), based on the nonlinear resonance between a known inductor and the output capacitance of the device under test. The method is simple and robust, and only requires a single voltage measurement to extract the large-signal C OSS both in charging and discharging transients. By changing the circuit parameters, it is possible to tune the resonance frequency (even above 40 MHz) and the voltage swing (even above 1 kV) with d v /d t exceeding 100 V/ns, even though the method relies only on a low-voltage dc source, without the need for high-voltage RF amplifiers. The single-pulse operation of the method enables measuring C OSS and E DISS at very high frequency and d v /d t values without any thermal runaway. Using the proposed method, we extracted large-signal C OSS and E DISS of power transistors based on different semiconductor technologies. The obtained results were verified by Sawyer–Tower method and data reported in the literature. In particular, dv/dt-independent C OSS losses were measured in cascode GaN transistors. The precise characterization of large-signal C OSS of transistors presented in this letter is essential for the design of power converters, especially those operating at high switching frequencies.

Anomalous photovoltaic effect based on molecular chirality: influence of enantiomeric purity on the photocurrent response in π-conjugated ferroelectric liquid crystals.

(S)- and (R)-forms of chiral π-conjugated ferroelectric liquid crystals were synthesized. The dielectric properties in the ferroelectric liquid-crystalline (FLC) phases were evaluated using the Sawyer-Tower method. Spontaneous polarization of (S)-1 reached 68 nC cm-2 at 127 °C. Hole mobilities in the FLC phases estimated using the time-of-flight (TOF) method were on the order of 10-4 cm2 V-1 s-1. Each chiral π-conjugated compound exhibited a photovoltaic effect based on spontaneous polarization without p-n or Schottky junctions. This phenomenon could be attributed to an anomalous photovoltaic (APV) effect that has been observed in ferroelectric ceramics. In addition, liquid-crystalline enantiomeric mixtures of (S)-1 and (R)-1 were prepared and the APV response under UV illumination was studied. The APV response was enhanced with an increase in enantiomeric purity and was minimized in the racemic mixture. From this result, it was concluded that the APV effect in this FLC compound originated from the molecular chirality.

Multiferroic behavior of disordered bismuth-substituted zinc ferrite

In this study, we reported the multiferroic behavior of disordered bismuth-substituted zinc ferrite. Bi:ZnFe2O4 films were deposited by using the RF-magnetron sputtering method. As-deposited films exhibited a ferromagnetic hysteresis and a large Faraday rotation angle in the short wavelength range (∼400 nm): a saturation magnetization of 120 G and the Faraday rotation angle of −1.2 deg/μm were observed. Ferroelectric hysteresis was measured using the Sawyer-Tower method. Dielectric constant of Bi:ZnFe2O4 was measured with a LCR meter in a frequency range from 120 Hz to 100 kHz. For 120 Hz, the dielectric constant of 800 was observed at room temperature. By contrast, the dielectric constant of disordered zinc ferrite was 5.5 at 120 Hz. Our studies showed that bismuth substitution in ZnFe2O4 enhanced the dielectric constant drastically. Furthermore, in the films, voltage-induced alteration of the magneto-optic Kerr rotation angle was identified.

Structural and Dielectric Properties of Glycine Phosphite Thin Films

Thin polycrystalline films of amino acid ferroelectrics glycine phosphite (GPI) have been prepared by evaporation method on NdGaO3(001) substrate with interdigital structure of electrodes deposited on the surface. Optical images of films obtained by means of polarized microscope in reflection mode have shown that the films consist of small single crystal blocks of several tens of microns in size. X-ray diffraction analysis proves that the non-polar c* axis in blocks coincides with the normal to the surface, and the polar axis b is oriented in the film plane. Temperature dependences of capacitance of GPI/NdGaO3 structures measured using interdigital structure of electrodes exhibit strong anomaly at ferroelectric phase transition temperature of glycine phosphite single crystals. Dielectric hysteresis loops in glycine phosphite films were measured by Sawyer-Tower method. The difference in frequency dependence of coercive field and remnant polarization in structures with GPI and previously studied betaine phosphite films is analyzed and attributed to considerable difference in dimensions of blocks in these structures.

Small- and strong-signal dielectric response in a single-crystal film of partially deuterated betaine phosphite

Poly- and single-crystal films of betaine phosphite deuterated to ∼20% have been grown by evaporation on NdGaO3 (001) substrates with a preliminarily deposited planar interdigital structure of electrodes. The small-signal dielectric response in the 0.1–100.0-kHz frequency range has revealed a strong anomaly in capacitance upon the transition of the films to the ferroelectric state. Application of a bias field brings about suppression and a slight shift of the dielectric anomaly toward higher temperatures. The strong-signal dielectric response has been studied by the Sawyer-Tower method over the frequency range 0.06–3.00 kHz both in the para- and ferroelectric phases. In contrast to the case of a plane-parallel capacitor, in the planar structure studied, the dielectric hysteresis loops exhibit a very small coercivity at low frequencies, which grows with increasing frequency. This difference should be assigned to different domain structures formed in a planeparallel capacitor and in a planar structure in a saturating field. The growth of hysteresis with increasing frequency in a planar structure is considered to be associated with the domain wall motion.

Study of properties of nanoscale lead zirconate titanate films

Characteristics of lead zirconate titanate (PZT) alloy films 50–200 nm thick grown on sapphire and silicon substrates by low-temperature synthesis from Ti52Zr48 alloy films were studied. The surface morphology and composition of PZT films were studied and some electrical characteristics of the obtained coatings were determined. The occurrence of the piezoelectric effect in films was observed by the Sawyer-Tower method, as was the dependence of the shape of the hysteresis loop on the value and frequency of the applied voltage.

The use of the Sawyer-Tower method and its modifications to measure the electrical parameters of ferroelectric materials

Modifications of the Sawyer-Tower method, based on a measurement of the impedance and Fourier transformations, are proposed. A functional and metrological analysis of the methods is presented, which enables the reliability of the measurements of the field dependences of the parameters of ferroelectric materials to be increased, which should find application in microelectronic and nanoelectronic products.

Switching Dynamics in Ferroelectric Liquid Crystal Mixture

Switching dynamics have been investigated in homogeneously aligned 5 µm thin cell filled with (FLC-6980) ferroelectric liquid crystal mixture using field reversal method and Sawyer Tower method. The effect of applied voltage and temperature on switching time has been studied. It has been observed that switching time decreases with the increase of applied voltage and also with the rise of temperature. The tortional viscosity has been calculated. It shows Arrhenius type behavior. Hysteresis loop has been studied at different temperatures and at different frequencies. It has been observed that coercive field (Ec) increases with the increase of frequency. Ec has non-zero value at zero frequency, which is important characteristics of ferroelectrics. Ec increases near the transition temperature.

Mn and Sm doped lead titanate ceramic fibers and fiber/epoxy 13 composites

Manganese and samarium doped lead titanate [Pb0.85Sm0.10(Ti0.98Mn0.02)O3, PSmT] fibers were prepared by sol-gel method. The micrographs obtained using scanning electron microscope show that PSmT ceramic fibers are round and dense. The diameter of the fibers was in the range of 30–35 μm. The crystalline grains size is ∼2.5 μm. The micrographs obtained using transmission electron microscope also unveiled the layer-by-layer 90° domains in the grains. X-ray diffraction patterns of the fibers show that PSmT ceramics have a pure perovskite structure. The c/a ratio of the unit cell was 1.04. The PSmT fiber/epoxy 1-3 composites were fabricated by filling the ceramic fiber bundle with epoxy. The dielectric permittivity ε, electromechanical coefficient kt, and the piezoelectric constant d33 of PSmT fiber/epoxy 1-3 composites with 68% fiber loading were 118, 0.51, and 48 pC/N, respectively. The hysteresis loop of the composites was measured by the Sawyer–Tower method. It was also found that the composites could withstand an electric field of 15 kV/mm at room temperature.

Dynamic response and hysteresis dispersion scaling of ferroelectric SrBi2Ta2O9 thin films

The dynamic hysteresis response of ferroelectric SrBi2Ta2O9 thin films versus periodically varying electric field over a frequency range of f=10−1–106 Hz and amplitude range of E0=15–158 kV/cm is measured utilizing the Sawyer–Tower method. The dynamic order parameter Q shows anomalous behavior against the field amplitude, and a single-peaked hysteresis dispersion is identified. The field response of hysteresis area 〈A〉 in the form of 〈A〉∝f2/3E02/3 over the low frequency range is evaluated, while the response over the high frequency range takes the form of 〈A〉∝f−1/3E02. We demonstrate that the hysteresis dispersion spectrum exhibits single-parameter scaling, and predicts a characteristic time for domain reversal that is inversely correlated to the field amplitude.

Pyroelectric Properties of (Pb,Ca)[(Co1/2W1/2)Ti]O3 Ceramics.

The pyroelectric properties of (Pb1-x, Cax) [(Co1/2W1/2)Ti]O3 ceramics have been studied by substituting Ca for Pb and by measuring spontaneous polarization using the Sawyer-Tower method in order to clarify the mechanism responsible for the improvement of pyroelectric property. With an increase in the Ca concentration range of x=0.20-0.30, spontaneous polarization, Ps increases and reaches large values of 40-42μC/cm2 at 100°C, as compared to the 2-3.5μC/cm2 of the representative PbTiO3 ceramics containing small amounts of La and Mn. The Curie temperature decreases according to the Vegard law. The coercive electric field Ec markedly decreases from 28kV/cm at a Ca concentration of x=0.10 to 4kV/cm at a Ca concentration of x=0.40, compared with the 60kV/cm of PbTiO3 ceramics. Increasing spontaneous polarization might be explained in terms of lattice distortion by Ca ions. It is found that a significant improvement of pyroelectric properties is related to an increase in spontaneous polarization Ps and a decrease in the Curie temperature.

Abnormal ferroelectric properties of compositionally graded Pb(Zr,Ti)O3 thin films with LaNiO3 bottom electrodes

We report abnormal ferroelectric properties of compositionally graded Pb(Zr,Ti)O3 thin films on LaNiO3-coated SiO2/Si substrates, where Pb(Zr,Ti)O3 and LaNiO3 films were prepared by a metalorganic decomposition technique and a sol–gel technique, respectively. It was found that the hysteresis loops of the Pb(Zr,Ti)O3 graded films measured by the conventional Sawyer–Tower method shifted along the polarization axis, i.e., they showed polarization offsets when applied by an alternating electric field. The polarization offsets were 82.5 μC/cm2 at 270 kV/cm and 62.5 μC/cm2 at 185 kV/cm for the up-graded film and the down-graded film, respectively. The absolute magnitude of the polarization offsets was closely related to the magnitude of the driving electric field, and the direction of the polarization offsets depended on the direction of the composition gradient with respect to the substrate. Analysis indicated that the polarization offsets did not originate from asymmetric contact effects, oxygen vacancies, alignment of defect dipoles, and/or electron injection. These results showed that polarization offsets in hysteresis loops were the intrinsic characteristic of the compositionally graded Pb(Zr,Ti)O3 thin films.