Increasing demands for power supplies have contributed to the population of high-frequency ac (HFAC) power distribution system (PDS), and in order to increase the power capacity, multilevel inverters ...(MLIs) frequently serving as the high-frequency (HF) source-stage have obtained a prominent development. Existing MLIs commonly use more than one voltage source or a great number of power devices to enlarge the level numbers, and HF modulation (HFM) methods are usually adopted to decrease the total harmonic distortion (THD). All of these have increased the complexity and decreased the efficiency for the conversion from dc to HF ac. In this paper, a nine-level inverter employing only one input source and fewer components is proposed for HFAC PDS. It makes full use of the conversion of series and parallel connections of one voltage source and two capacitors to realize nine output levels, thus lower THD can be obtained without HFM methods. The voltage stress on power devices is relatively relieved, which has broadened its range of applications as well. Moreover, the proposed nine-level inverter is equipped with the inherent self-voltage balancing ability, thus the modulation algorithm gets simplified. The circuit structure, modulation method, capacitor calculation, loss analysis, and performance comparisons are presented in this paper, and all the superior performances of the proposed nine-level inverter are verified by simulation and experimental prototypes with rated output power of 200 W. The accordance of theoretical analysis, simulation, and experimental results confirms the feasibility of proposed nine-level inverter.
In this paper, a quasi-resonant switched-capacitor (QRSC) multilevel inverter (MLI) is proposed with self-voltage balancing for single-phase high-frequency ac (HFAC) microgrids. It is composed of a ...QRSC circuit (QRSCC) in the frontend and an H-bridge circuit in the backend. The input voltage is divided averagely by the series-connected capacitors in QRSCC, and any voltage level can be obtained by increasing the capacitor number. The different operational mechanism and the resulting different application make up for the deficiency of the existing switched-capacitor topologies. The capacitors are connected in parallel partially or wholly when discharging to the load, thus the self-voltage balancing is realized without any high-frequency balancing algorithm. In other words, the proposed QRSC MLI is especially adapted for HFAC fields, where fundamental frequency modulation is preferred when considering the switching frequency and the resulting loss. The quasi-resonance technique is utilized to suppress the current spikes that emerge from the instantaneous parallel connection of the series-connected capacitors and the input source, decreasing the capacitance, increasing their lifetimes, and reducing the electromagnetic interference, simultaneously. The circuit analysis, power loss analysis, and comparisons with typical switched-capacitor topologies are presented. To evaluate the superior performances, a nine-level prototype is designed and implemented in both simulation and experiment, whose results confirm the feasibility of the proposed QRSC MLI.
In this paper, switched Z-source/quasi-Z-source dc-dc converters (SZSC/SQZSCs) are proposed for the photovoltaic (PV) grid-connected power system, where the high step-up dc-dc converters are required ...to boost the low voltage to high voltage. The boost factor is increased by adding another one switch and diode to the output terminals of traditional Z-source/quasi-Z-source dc-dc converters. Not only does the output capacitor function as the filter capacitor; it is also connected in series into the inductors' charging loops when both switches are turned on. Compared with existing Z-source based structures, higher boost factor is realized through a small duty cycle (smaller than 0.25). On the one hand, the instability caused by the saturation of the inductors can be avoided. On the other hand, a larger range can be reserved for the modulation index of the backend H-bridge when they are used for the dc-ac conversion. Moreover, much fewer passive components are employed when compared with the recently proposed hybrid 3-Z-network topologies that have the same voltage gain, which can enhance the power density and decrease the cost. The performances of the proposed converters, including their operational principles in continuous and discontinuous current modes, voltage and current parameters of components, and impacts of parasitic parameters, are analyzed. The simulation and experimental results are given to verify the aforementioned characteristics and theoretical analysis.
A series of dithienylethene (DTE) photoswitches with aza‐heteroaromatic cationic moieties is synthesized. The switches are characterized regarding their photochemical and photophysical properties in ...acetonitrile and in water. The efficiency of the switching and the photostationary state composition depend on the degree of π‐conjugation of the heteroaromatic systems. Thus, DTEs with acridinium‐derived moieties have very low quantum yields for the ring‐closing process, which is in contrast to switches with pyridinium and quinolinium moieties. All switches emit fluorescence in their open forms. The involved electronic transitions are traced back to an integrative picture including the DTE core and the cationic arms. The emission can be fine‐tuned by the π‐conjugation of the heteroaromatic cations, reaching the red spectral region for DTEs with acridinium moieties. On ring‐closing of the DTEs the fluorescence is not observable anymore. Theoretical calculations point to rather low‐lying energy levels of the highly conjugated ring‐closed DTEs, which would originate near‐infrared emission (> 1200 nm). The latter is predicted to be very weak due to the concurrent non‐radiative deactivation, according to the energy‐gap law. In essence, an ON–OFF fluorescence switching as the result of the electrocyclic ring‐closing reaction is observed.
Dithienylethenes are structurally integrated with heteroaromatic cations. The photoswitches in their ring‐open forms show fluorescence in the visible spectral range, while the ring‐closed isomers are non‐fluorescent. This leads to ON–OFF fluorescence switching with the emission output being fine‐tuned by the conjugation degree of the heteroaromatic units.
A novel single-phase nine-level switched-capacitor inverter (9LSCI) is presented with quadruple boost ability and reduced components. The proposed topology with single dc source employs only eight ...switches to realize nine-level output, self-voltage balance of capacitors, quadruple boost and inductive-load ability, thus, the effective cost is cut down compared to other switched-capacitor multilevel inverters (SCMLIs). Different from other SCMLIs, the proposed 9LSCI has no need for back-end H-bridge, of which four switches need to withstand the peak voltage of output. Hence, total standing voltage can be reduced. The operation principles containing the self-voltage balance of capacitors are described in detail. The quantitative comparisons, modified cost function, as well as the loss evaluations are examined in depth. Finally, multicarrier phase disposition pulsewidth modulation method is adopted and a laboratory prototype is implemented with the rated output of 220 V-500 W. The experimental results also verify the feasibility of the proposed topology.
In conventional hybrid multilevel inverters (MLIs), the voltage balance controls of floating capacitors are usually realized by redundant switching states using high-frequency modulation. The ...switching frequency, as well as the switching loss, is hence increased, and the topologies are limited to low-frequency output occasions as a result. In switched-capacitor MLIs (SCMLIs), the capacitors are self-balanced, since they are charged to fixed voltage levels by the input source directly or indirectly. By integrating switched-capacitor techniques into conventional hybrid MLIs, a pair of symmetric/asymmetric hybrid MLIs is proposed in this paper with boost ability. Complicated balance controls are avoided for capacitor voltages, and thus the modulation gets simplified. Compared with SCMLIs, the backend H-bridges that withstand the accumulated voltage stress are removed. The peak inverse voltages of devices are decreased, and thus only low-voltage components are needed. Meanwhile, the component numbers are reduced greatly. All these advantages contribute to high efficiency and reduced costs. Consequently, the proposed hybrid MLIs are especially appropriate for synthesizing a staircase output directly from an input source of low voltage. Analyses and their operational principles are firstly presented for the proposed topologies. Comparisons are then carried out to illustrate their superiority. To evaluate the performance, simulation and experimental prototypes are implemented, whose results confirm the feasibility of the proposed hybrid MLIs.
The Hebrew Bible (James, 2011), as a collection of ancient books of history, prophecies, poems and wisdom literatures, contains some of the most intriguing stories and profound wisdom from the ...ancient world. While majority of these ideas and sapiential teachings have already deeply integrated into our modern cultures, there are yet some that we still struggle in understanding and accepting, such include the origin of sin and evil in the story of Adam and Eve in the garden of Eden and the story of Job with his undeserved sufferings. In this study, we are going to conduct a comparative study of the two biblical stories mentioned based on resources in and out of the Bible. We hope that our study will be able to cast some new lights over these long-discussed topics and to help our readers in their own reading experience.
•Dense strontium ferrite parts were successfully prepared by direct ink writing (DIW).•The printing lines were staggered and overlapped stacking for DIW.•The DIW remarkably improves the density and ...mechanical strength of SrFe12O19 parts.•The stacking method provides a novel technology for the densification of DIW.
An innovative process was proposed to solve the limitations of direct ink writing (DIW) in the manufacture of dense ceramics. Dense strontium ferrite parts were successfully prepared by DIW. This innovative process that the staggered and overlapped stacking effectively eliminated the gaps among adjacent printing filaments, which could also be applied to other additive manufacturing technologies. The density and mechanical properties of samples were remarkably improved based on theoretical calculation and program setting. Results show that compared with staggered stacking method, the relative density of samples prepared by staggered and overlapped stacking method increased from 97.42 ± 0.20% to 99.35 ± 0.20%, and the surface roughness reduced from 11.42 ± 0.50 μm to 5.48 ± 0.50 μm. The three-point bending strength and compressive strength of sintered samples were 84.00 ± 5.00 MPa and 610.00 ± 20.00 MPa, respectively.
Cu-Se compounds have attracted extensive attention in the field of thermoelectric field, especially for the Cu-Se film material. Cu content will affect the conductivity and Seebeck coefficient of the ...Cu-Se films. Thus, the Cu-Se films with Cu contents in the range of 58%–90% are prepared by vacuum thermal evaporation to explore the effect of Cu contents on microstructure and electrical properties. The phase composition changes from CuSe phase to Cu2Se phase and then to the coexistence of Cu phase and Cu2Se phase with increasing the Cu contents. In various Cu contents, the Cu0.68Se0.32 film is mainly formed by β-Cu2Se phase and presents smooth and dense surface. This makes the film have lower resistivity and the highest Seebeck coefficient than the cases with other Cu contents. The film has maximum power factor of 770 μW m-1 K-2 at 250 °C when the atomic ratio of the film is close to 2:1.
In this study, porous calcium silicate (CaSiO3) scaffolds were prepared by 3D gel-printing (3DGP) method and Fe3O4 water-based magnetic fluids (WMFs) were prepared by phacoemulsification compound ...chemical coprecipitation method. Fe3O4 WMFs were coated on CaSiO3 scaffolds surface to prepare Fe3O4/CaSiO3 composite scaffolds. The effect of WMFs with different Fe3O4 concentrations on porous CaSiO3 scaffolds was studied. The composition and morphological characteristics of porous scaffolds were analyzed by using scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) analysis. The magnetic properties were tested by vibrating sample magnetometer (VSM). The stability of Fe3O4 WMFs coatings and the degradability of composite scaffolds were tested by immersing them in simulated body fluid (SBF). The results show that when Fe3O4 concentration was 5.4% (w/v), the composite scaffolds had the highest saturation magnetization of 69.6 emu/g and the best stability in dynamic SBF. It is obviously that Fe3O4 WMFs coatings can be used for bone tissue engineering scaffolds repairing.
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