Synthesis of lanthanum–strontium manganites by oxalate-precursor co-precipitation methods in solution and in reverse micellar microemulsion

Nanostructured lanthanum–strontium manganites were synthesized using two different co-precipitation approaches, one in bulk solution, and the other in reverse micelles of CTAB/1-hexanol/water microemulsion. In both cases, precursor cations were precipitated by using oxalic acid. The properties of the materials synthesized by using these two methods were compared in order to reveal potential advantages of the microemulsion-assisted approach. The influence of the annealing conditions on the properties of synthesized manganites was investigated by using X-ray diffraction, transmission electron microscopy, differential thermal analysis, thermogravimetric analysis and magnetic measurements.     

The synthesis of iron–nickel alloy nanoparticles using a reverse micelle technique

Nanosized Fe_0.2Ni_0.8 particles were prepared by reducing their salts with sodium borohydride (NaBH₄) in cationic water-in-oil (w/o) microemulsions of water/cetyl-trimethyl-amonium bromide (CTAB) and n-butanol/isooctane at 25 °C. According to the TEM and X-ray diffraction analyses, the synthesized particles were around 4–12 nm in size. Due to their nanodimensions, the particles had a primitive cubic (pc) structure rather than the body-centered cubic (BCC) structure of the bulk material. An examination of the synthesis from the reverse micelle reveals that the morphology of the iron–nickel alloy nanoparticles depends mainly on the microemulsion's composition. The magnetization of the nanoparticles was much lower than that of the bulk material, reflecting the influence of the nanodimensions on the particles’ magnetizations.     

Discontinuous Space-Vector Modulation for Three-Level PWM Rectifiers

This paper presents the implementation and experimental verification of two discontinuous pulsewidth modulation (DPWM) methods for three-phase, three-level rectifiers. DPWM's features, such as improved waveform quality, lower switching losses, reduced ac-side passive component size, are investigated and compared to the conventional continuous pulsewidth modulation (CPWM). These features allow higher power density and/or efficiency to be achieved and are important targets for the next generation of power rectifiers. The implementation of the two DPWM strategies is explained by means of space-vectors representation and modulation functions. A detailed analysis of both ac-side and dc-side current waveforms is presented, and there is excellent agreement between the analytical, simulated and experimental results for the mains current ripple amplitude and output center-point current over the practical modulation range. Finally, the control of the center-point voltage is discussed.     

电力电子技术中虚拟原型和综合多目标优化技术研究(上)

新的拓扑结构,新的调制方法和未来的宽能带隙的半导体器件技术将会满足电力电子系统更高效率,更高功率密度和更低成本的要求。但是,在设计过程中通过使用多范畴/多目标优化,亦即对设计变量都用最优值,也能显著地提高目前技术系统的性能。为了实现这些优化,首先需要对主电路建立一个综合的数学模型,包括热模型,DM和CM EMI滤波器模型。基于这些模型,可以得到诸如效率,功率密度等的多目标优化。优化可以充分地使用所有的设计自由度,也允许确定根据诸如功率半导体器件的品质因数或磁芯材料的性质等基本技术决定的系统性能的敏感度。而且,能够容易地比较不同拓扑结构和识别固有性能的限制。本文首先描述设计一个电力电子变换器的主要功能要素的分析途径和一个线性设计过程。接着连接诸如电、磁、热和热-机设计范畴的元件模型,讨论基于这些连接模型的各个设计变量的优化。最后,研究不同范畴的耦合和实施这些耦合的等效电路的利用实例。     

Synthesis and structure of hydroxylammonium fluoroaluminate

Abstract  

Hydroxylammonium fluoroaluminate with the formula (NH₃OH)₂AlF₅ was synthesized by the reaction of solid NH₃OHF and an aqueous solution of aluminum in HF, and its structure was determined by single-crystal X-ray diffraction. The structure consists of NH₃OH⁺ cations and centrosymmetrical AlF₆ octahedra, which are formed by sharing two opposite corners connected in polymeric (AlF₅)_n²⁻ anions. Oxygen and nitrogen atoms of hydroxylammonium cations are donors of hydrogen bonds to terminal fluorine atoms of fluoroaluminate chains. The compound crystallizes monoclinic P2/c, with cell parameters a = 10.8675(3) ?, b = 7.3098(2) ?, c = 7.2071(2) ?, and β = 91.080(2)°. The thermal decomposition of the compound was studied by TG, DSC, and X-ray powder diffraction. By controlled heating of (NH₃OH)₂AlF₅, a new compound with the formula (NH₃OH)AlF₄ was obtained, and the final product of the decomposition is γ-AlF₃.     

Pump Characteristic based optimization of a direct water cooling system for a 10-kW/500-kHz Vienna rectifier

An ultra high power density 10-kW/500-kHz three-phase pulse-width modulation rectifier (Vienna Rectifier) is under development at the Power Electronic Systems Laboratory, ETH Zurich. From preliminary measurements and numerical simulations the total efficiency is assumed to be 95% at full load, resulting in power losses of up to 150 W in each multichip power module that realizes a bridge leg of the rectifier. In order to maintain the required power density of the system high direct water cooling is employed where water is in direct contact with the module base plate. Based on the measured characteristic of the water pump (pressure drop dependent on the water flow rate) the geometry of different water channel structures below the module base plate is systematically optimized based on equations which are formulated using well-established fluid dynamics theory. The design optimization is constrained by the desire to keep the geometry of the water channels in a range which allows simple and low-cost manufacturing. The aim is to find a channel structure resulting in a minimum thermal resistance of the power module for a given pump characteristic. In this paper, a very simple slot channel is investigated. The dependency of the thermal resistance on the cooling system is calculated for various heights of the slot channel, and an optimized channel height is determined using the condition of simple manufacturability. The shortcomings of the simple slot structure are discussed, and a novel metallic inlay structure is introduced and optimized that results in a reduction of the thermal resistance of the direct water cooling scheme as compared to the slot channel system. All theoretical considerations are experimentally verified. The general optimization scheme introduced in this paper can easily be adapted to other cooling problems.     

Sonochemically assisted synthesis of zinc-doped maghemite

Nanoparticles of zinc-doped maghemite were prepared using ultrasonic radiation. As a precursor, a suspension of maghemite in an alkaline aqueous solution of zinc nitrate at pH 9 was sonicated. The zinc-doped maghemite nanoparticles were investigated by X-ray diffraction, Mössbauer spectroscopy, high-resolution electron microscopy (HREM) and SQUID magnetometry. The Mössbauer measurements, which cover the temperature range 4.2 K to room temperature, were acquired in zero field and an applied field of 5 T. The results show that by using ultrasound radiation, zinc Zn²⁺ can substitute for Fe³⁺ up to a composition close to zinc ferrite (ZnFe₂O₄), which has a random distribution of Fe³⁺ ions over both A and B sublattices in the spinel structure with an inversity parameter of δ = 0.322. This leads to a maximum saturation magnetization (M_s) of 64.1 emu/g at 300 K and 73.5 emu/g at 2 K.     

Electromagnetic wave absorption of polymeric nanocomposites based on ferrite with a spinel and hexagonal crystal structure

We have investigated composites designed for microwave absorption based on magnetic filler, composed of phases within the SrO-Fe₂O₃ system, embedded in a polyphenylene sulfide matrix with a concentration ratio of 80:20 by weight. The formation of the nanosized particles of SrFe₁₂O₁₉ and Fe₃O₄, as the principal magnetic phases was achieved via the co-precipitation of Sr²⁺/Fe³⁺ ions using different molar ratios. The various precursors obtained were calcined between 600 °C and 900 °C in air. The electromagnetic parameters of the composites were measured with a vector network analyzer at 400 MHz to 32 GHz. The results show that with a composite composed of a complex magnetic filler comprising the nanoparticles of two magnetically diverse phases, i.e., a spinel phase as the electromagnetic wave absorber in the lower GHz range and a hexagonal phase operating at a higher GHz range, above 32 GHz, a microwave absorber with an broad absorption range can be prepared.     

Vergleich, Dimensionierung und Realisierung einphasiger Power Factor Correctors minimalen Realisierungsaufwands

In the future laws and restrictions will limit the amplitudes of the low frequency harmonics of mains currents of rectifier systems. While diode bridges are very simple, compact and cheap, their input current shows high low-order harmonics. In the past various rectifier systems have been developed that avoid low-order input current harmonics by means of active current control. The significantly reduced input filter results in a small and compact total rectifier system. In this paper different rectifier systems are discussed and compared. Since industry is interested in simple and cheap solutions the rectifier systems are discussed under these considerations. Besides an introduction of different power circuits and control schemes, the systematic choice of the power semiconductors is presented.