Volume 61 (2010): Issue 5 (September 2010)

This work relates to the study of the vector control of the salient-pole double star synchronous machine drive (DSSM) fed by three-level inverters which are controlled by PWM hysteresis strategy. Indeed, it is a question of carrying out a decoupling between rotor flux and electromagnetic torque, by introducing a vector control with an optimal torque working and by imposing constant flux regime. Furthermore, to ensure a decoupled dynamic behavior of the machine as in the case of a DC machine, a regulation of the rotor current has been introduced through a chopper operating at 5 kHz chopping frequency, feeding the excitation circuit. The obtained results are very satisfactory and reveal the effectiveness of the proposed approach.

Nanocrystalline soft-magnetic ribbons promised a wide-spread practical use when introduced at the beginning of nineties. After 20 years of extensive research there are still unclear material problems which are thought to be the principal reason why these materials show but marginal use. Poorly controllable magnetic anisotropy due to spontaneous intrinsic macroscopic stress that comes from an inevitable heterogeneity of the ribbon materials is pointed to in this work. Certain stress-based mechanisms are shown to induce the unintended anisotropy in the already familiar Finemets as well as in the newer Hitperms. Hysteresis loops, domain structure and power loss is used to reveal the anisotropy consequences and particular connected but still unanswered questions are pinpointed.

Thin-film silicon solar cell technology is one of the promising photovoltaic technologies for delivering low-cost solar electricity. Today the thin-film silicon PV market (402MW_p produced in 2008) is dominated by amorphous silicon based modules; however it is expected that the tandem amorphous/microcrystalline silicon modules will take over in near future. Solar cell structures based on thin-film silicon for obtaining high efficiency are presented. In order to increase the absorption in thin absorber layers novel approaches for photon management are developed. Module production and application areas are described.

In this paper a steady state and dynamic assessment of electrical shaft with common resistance are performed. New steady state formulae that take into account the stator resistances are developed. The system is presented in a full version similar to individual induction machine in the dq frame but without stator resistance simplifications. It is found that the stator resistance is worth-taking into account.

The dynamics of a magnetic domain wall (DW) in a wire is studied. The DW is modeled as a Brownian particle subjected to thermal fluctuations and is characterized by the mass, position and velocity. Its motion is damped by friction, pinned by the irregularities in the material and driven by a constant force due to the external magnetic field. We have obtained the corresponding Langevin equation that contains a white-noise force. The use of an effective method taken from the statistical physics allowed us to convert this stochastic equation into an ordinary differential equation. From its solution the mean square displacement of the DW with other relevant time correlation functions and their spectral densities have been found. The electric current induced by the moving DW is also calculated.

In this paper we present the current work and experience with using microfocus x-ray generator and commercial CCD camera for x-ray imaging purpose. There is a need in laboratories for the development of imaging methods approaching synchrotron radiation sources, where the brilliance of radiation is on very high-level. Generally, there is no continuous access to synchrotron facilities. Several synchrotron radiation laboratories allocate the access via a proposal system. Thus the time for synchrotron radiation experiments seldom exceeds more than 1-2 weeks per year, which restricts its application to a few selected experiments. Even in future, the routine characterization of samples will be performed mainly at the experimenters home laboratories [10]. In this contribution we show that with the present set-up it is possible to achieve the spatial resolution down to μm and with the appropriate geometry a phase contrast images are observable.

A novel approach to the preparation of thin films by using RF diode sputtering was developed. In one deposition run, thin films of doped ZnO:(Al, Ga) were obtained with different structural, optical and electrical properties. ZnO: Al films of low resistivities in order of 10^-3Ωcm and high optical transmittances about 93% have appeared the most suitable for photovoltaic applications.

This experimental work deals with measuring Raman spectra of rubrene. The objective is to optimize the measurement procedure of rubrene layers on a substrate. The main outcome of the work is identification of rubrene and of the single-crystalline nature of the measured spots of the rubrene layer.

Chemical synthesis of binary Fe-Co oxalate was used for preparation of Fe-Co nanoparticles. X-ray difftaction spectra recorded during temperature treatment in reduction atmosphere show two main stages of transformation: amorphization of the oxalate mixture and formation of Fe-Co nanoparticles. After X-ray diffraction high temperature treatment the nanoparticles with 50 nm mean crystallite were found. Comparison of magnetic properties during and after different type of temperature treatment shows that coercivity and saturation magnetic polarization are sensitive in dependence on sample condition, length of treatment and temperature. Analysis of Mössbauer spectra reveal several components which were ascribed different phases during temperature treatments.

At present, great attention is given to study of preparation and properties of various nanomaterials usable in many applications. They are utilized in varied fields of human activity - eg in electronics, medicine, paint industry etc. Besides the detailed chemical structure, such nanoparticle properties as the shape and size distribution are fundamental to the given application. To measure these parameters various methods are used, e.g. transmission electron microscopy (TEM), atomic force microscopy (AFM), acoustic spectrometry, methods based on the light scattering and X-ray disc centrifuge system.

The indentation load-size effect (ISE) in Vickers hardness of Al₂O₃ and Al₂O₃ + SiC nanocomposites has been investigated and analysed using Meyer law, proportional specimen resistance (PSR) model and modified proportional specimen resistance (MPSR) model. The strongest ISE was found for alumina. Both the PSR and MPSR models described the ISE well, but the MPSR model resulted in slightly lower true hardness values for all materials investigated. No evidence of the effect of machining stresses on the ISE has been found.

The main aim of this paper is to show how ECAP influences the densification behaviour of PM aluminium alloys. An aluminium based powder (Al-Mg-Si-Cu-Fe) was used as material to be investigated. After applying different compacting pressures, specimens were dewaxed in a ventilated furnace at 400 °C for 60 min. Sintering was carried out in a vacuum furnace at 610 °C for 30 min. The specimens were ECAPed for 1 pass. Optical characterization was carried out on the minimum of 10 different image fields. The results were measured for each pore individually in order to describe the dimensional and morphological porosity characteristics. ECAP influences the porosity distribution in terms of the severe shear deformation involved.

Carbon nanostructures present the leading field in nanotechnology research. A wide range of chemical and physical methods was used for carbon nanostructures synthesis including arc discharges, laser ablation and chemical vapour deposition. Plasma enhanced chemical vapour deposition (PECVD) with its application in modern microelectronics industry became soon target of research in carbon nanostructures synthesis. Selection of the ideal growth process depends on the application. Most of PECVD techniques work at low pressure requiring vacuum systems. However for industrial applications it would be desirable to work at atmospheric pressure. In this article carbon nanostructures synthesis by plasma discharges working at atmospheric pressure will be reviewed.

This feature article gives an overview of recent advances in development of high performance molecular organic semiconductors for field-effect transistors with emphasis on the structure of molecular materials and requirements for high-performance.