No. 4, 2007


Jan Gruber, Jan Hlina, Jiri Sonsky : Comparison of plasma torch dynamics in reconstructed phase space for different input parameters, pp. 321-329

Method of time-embeddings is used for reconstruction of plasma torch dynamics. Different input currents and argon flow rates have been used and the resulting dynamics of plasma flow compared. Electric arc inside the plasma torch and generated plasma jet were observed optically by arrays of photodiodes from 4 directions. Arc voltage and current were recorded simultaneously with the optical radiation. Reconstruction of process dynamics from both the records of arc current and plasma radiation by the method of time-embeddings was used to analyze the data. The results show that the changing character of the resulting phase portraits is related to the stability of working conditions reflected by the changes of the plasma jet axis position.

Tomas Chraska, Mariana Klementova, Jiri Hostomsky : Formation of nanocomposite alumina-zirconia-silica ceramics, pp. 331-341

Nanostructured ceramic materials were prepared from ternary Al2O3-ZrO2-SiO2 feedstock powders by plasma spraying and subsequent heat treatment of the rapidly solidified thick coatings or free standing parts. Three different feedstock powders with increasing SiO2 content were used in experiments. Resulting materials were characterized by a variety of analytical and microscopy techniques and selected mechanical properties were measured as well. The as-sprayed materials were amorphous or partially nanocrystalline depending on their SiO2 content. The as-sprayed material has low porosity and crystallizes when heated to approximately 950 C. By controlled annealing to crystallization temperature, nanocomposite structures were produced with predominant tetragonal ZrO2 nanocrystallites embedded in amorphous matrix. With increasing SiO2 content, the crystallization rate decreased and the average crystallite size increased. Furthermore, shrinkage on annealing decreased from 5 % to less then 2 % with increasing silica content.

Zygmunt Pitek : Magnetic pressure acting on a circular workpiece in the pulse magnetic field, pp. 343-356

In the paper we introduce a general formula describing the time-space distribution of the magnetic pressure in a conducting tubular workpiece placed in external magnetic field having the character of a damped sinusoid. For the case of a circular workpiece we determine the transient electromagnetic field using the solution of the Bessel equation in cylindrical coordinates, and also applying the integral Laplace transform. Next we determine the time-space distributions and instantaneous magnetic pressure inside the workpiece. The solution we obtained is illustrated by relevant graphs for different values of the parameter considering the external dimension and the depth of diffusion of the electromagnetic field.

Vlastimil Brozek, Viktor Sember, Milan Hrabovsky, Jan Janca, Jiri Domlatil : Photocatalysts activated by water stabilized plasma radiation, pp. 357-364

The changes in the photocatalytic activity of oxide semiconductors irradiated by the water stabilized DC arcjet of the plasma generator WSP are reported. The radiation power of the arcjet is 15 - 30 kW at maximum input power of 160 kW.

Petr Filip, Jiri David : Axial annular flow of power-law fluids - applicability of the limiting cases, pp. 365-371

A number of papers have aimed at an analytical solution of an axial annular flow of power-law fluids, especially at that of volumetric flow rate vs. pressure gradient. No complete analytical solution has been achieved as yet. The only analytical solutions-that have been hitherto derived-concern the limiting cases of a geometrical parameter k (inner-to-outer diameters ratio) or a flow behaviour index n. The present contribution discusses the applicability of these limiting solutions for a broader region of entry parameters and proves that usage of these relations is in many cases fully acceptable (and comparable with the inaccuracy in experimental determination of flow behaviour index n and consistency parameter k of a power-law model).

Karel Volenik, Oldrich Schneeweiss, Jan Pisacka, Blahoslav Kolman, Jiri Dubsky : Composition changes of a Fe-Al alloy during plasma spraying, pp. 373-384

A study of plasma spraying of a Fe-15.5 % Al alloy and of the composition changes accompanying this process is presented. The changes were determined by comparing the composition of a coating deposited by plasma spraying with that of the starting feedstock powder. In addition, the changes during the in-flight stage of the process before the particle impact onto the substrate were investigated. For this purpose, the flying molten particles were trapped and quenched in liquid nitrogen. The results show that rapid selective vaporization of aluminium occurred during the flight of molten particles in the plasma stream. The concentration of aluminium in the particles trapped in liquid nitrogen was considerably lower than in the feedstock powder. Vaporization affected the alloy composition more than selective oxidation. The only metallic phase in all samples was the Fe-Al solid solution, whereas no evidence of the presence of the ordered Fe3Al alloy was found.

Pavel Ctibor, Josef Sedlacek, Karel Neufuss : Dielectric properties of plasma sprayed silicates with and without additional annealing, pp. 385-397

Several silicate materials were plasma sprayed and characterized by the authors in recent years from the viewpoint of their sprayability, chemical and phase compositions, microstructure and mechanical as well as thermal properties. Present work is concerned with selected dielectric properties of these deposits.

Synthetic mullite and steatite as well as natural olivine-forsterite were plasma sprayed using the water-stabilized plasma system (WSP). The deposits were removed from substrate, ground and polished to produce samples in a shape of planparallel plates with a smooth surface. Part of sample series was later annealed in air. These samples-in principle monoblock capacitors-were then tested in the alternative low voltage electric field to measure capacity and loss factor in the frequency range from 200 Hz to 1 MHz. Relative permittivity was calculated from the measured capacity. Volume resistivity was measured in the direct electric field. It is shown that the relative permittivity of as-sprayed silicates is less stable (with frequency) compared to bulk in the whole studied frequency range. Annealing returns the structure closer to the equilibrium. This fact is reflected also by dielectric properties. Insulating ability of plasma-sprayed / annealed silicates is discussed in consequence with phase changes induced by annealing.

Pavol Spanik, Branislav Dobrucky, Michal Frivaldsky, Peter Drgona : Experimental analysis of commutation process of power transistor structures, pp. 399-413

The paper deals with testing device designed for experimental examination of processes in power electronics devices in various switching modes. Through the use of auxiliary circuits, additional switching modes (ZVS, ZCS) are realized except for hard switching and turning-off with reduced current. The advantage of the device is possibility of fine dead time setting, allowing us to analyze the effects of above-mentioned phenomenon on measurements of commutation losses.

Djemal Esaiashvili, Alexandre Akhvlediani : Application of the principle of the freedom of choice of possible displacements to the analysis of the stability of plastic systems with unilateral and bilateral links under impulsive seismic effect, pp. 415-422

The aim of the paper is to characterize the equilibrium under seismic loading of plastic systems with unilateral and bilateral joints.

It has been shown that the combination of the principle of possible Lagrange's displacement with D'Alembert's principle in case of nonstationary seismic loading of plastic system at the ultimate stage does not describe the terms of the system equilibrium, but it reveals the conditions of the loss of stability and collapse of the system.

It has been stated that a conventional design technique considering reduction coefficient to seismic loading is not sufficiently motivated. Artificial reduction of seismic loading design increases the risk of the loss of system stability.

The analysis of the damages to frame buildings under intensive earthquakes leads to the conclusion concerning the necessity of the behaviour factor, which would exclude transformation of building into plastic kinematic chain thus preventing collapse of its bearing elements following earthquake of intensity design.