Electron avalanches. I. Statistics of partial microdischarges in their pre-streamer stage

Three kinds of statistical distributions of DC microdischarges in homogeneous electric field are discussed. So far the exponential and streamer distributions have been considered as regular statistics describing microdischarge electron avalanches with low (n~<10/sup 5/) and high (n~>10/sup 8/) average carrier populations, respectively. The distributions of avalanches with intermediate populations n~/spl epsi/(10/sup 5/, 10/sup 8/) that exhibit peculiar convex shapes have not been considered as regular distributions so far. They have been regarded as an experimental artefacts causing deviations from the regular exponential law. On the basis of refined experiments we argue for a new interpretation of those peculiar distribution curves. They have proved to be independent of the remaining two regular statistics and manifest a new distribution law well approximated by the Pareto distribution. The basic physics underlying this separate statistical distribution is intimately related to the inner space charges formed inside the avalanches with higher carrier populations. The inner space charges that modify the local intensity of the electric field and result in redistribution of populations and manifestation of a new statistical behavior.     

Electron avalanches. II. Fractal morphology of partial microdischarge spots on dielectric barriers

For pt.I see ibid., vol.10, no.4, p. 689-699 (2003). Microdischarge spots appearing on surfaces of dielectric barriers in sandwiched electrode systems have been studied. The study has been focused on streamer microdischarges which are a subject of continual interest not only in plasma physics (dielectric barrier discharges) but also in HV technology (partial discharges). The microdischarge spots have been visualized using photographic films that have played the role of dielectric barriers in experiments. The digitized pictures of microdischarge spots have been subjected to multifractal analysis. An inner structure of spots has been revealed and ascribed to streamer heads. The monofractal nature of the overall sets of microdischarge spots as well as the monofractality of the borders of the dark spot centers has been indicated. The inner structure of the dark centers has been classified as a non-fractal planar object. The study represents a first attempt to analyze microdischarge spots on dielectric barriers using general multifractal formalism.     

Simplified digital acquisition of microdischarge pulses

A new concept of the measurement of the height of microdischarge pulses is proposed and implemented on a special hardware assembly. The concept utilizes the highly asymmetric shape of microdischarge pulses, i.e. the sharp rise edge and slowly decaying tail that forms an almost `horizontal' plateau. The maximum height of a pulse can be determined by imposing a certain time delay in saving sample points. The time delay must be equal to or longer than the time necessary for the full rise of the front edge of the pulse. The new concept completely removes time-consuming procedures seeking for signal local maxima used in current digitizers with logic circuits. This makes the system very fast and brings it to the very limit of the underlying natural process itself which represents the main advantage compared with other existing methods. A certain disadvantage of the new concept is in its restriction to asymmetric pulses only, but, however, this class of pulses comprises the broad spectrum of pulsating phenomena appearing in various branches of science and technology. This gives the proposed concept good applicability not only in the field of electric microdischarges but also in many other experimental domains     

Quasi-static compressive strength of cement-based materials

Wet cementitious materials show a noticeable dependence on the rate of quasi-static loading. While dry cementitious materials are almost independent of loading rate in the quasi-static region, the mechanical strength of wet materials increases with increasing rate of loading. Therefore, the Abrams' formula for the static mechanical strength cannot provide reliable values with wet materials at higher rates and should be corrected. Some possibilities for its improvement have been discussed.     

Some remarks on the dynamical conformity of rock joints

A recently developed computerized method for assessing the rock joint coefficients is discussed. The performances of formerly introduced relative similarity indicators, along with the correlation coefficient, are subjected to critical analysis. These relative numerical indicators are replaced by two absolute indicators whose properties better describe surface textures of rock joints. The first absolute indicator results from the Fourier Matrix and evaluates wavy shapes of surfaces. The second absolute indicator quantifies the heights of surface reliefs, and is defined as the root mean square height of the surface outline. The behavior of the newly introduced numerical indicators are investigated by means of the deterministic periodic surface reliefs. The practical application of the new indicators is presented and the convenient performances of both the indicators are documented.     

Ring Rolling Research at the Dresden University of Technology – its History from the Beginning in the 70s to the Present

The paper presents a brief history of ring rolling development at the Dresden University of Technology (DUT) from the 70s to the present. The technological scope covers the techniques explored at the DUT: Tangential profile ring rolling, axial profile tube rolling/roll plunging and the TRENPRO® method to produce shaped rings. Starting from the state of the art, technological principle, range of application and machine‐oriented implementation of these techniques are described. Furthermore, other common ring rolling variants, such as axial profile ring rolling, skew‐rolling and radial‐axial ring rolling, are taken into consideration. Finally, the authors provide a summary of problems still to be solved and ways to refine and disseminate the “ring rolling” discipline, assessing the rolling methods’ potential as extremely forward‐looking.     

Electrophysiological properties of neurones in cultures from postnatal rat dentate gyrus

Electrophysiological properties of neurofilament-positive neurones in dissociated cell cultures were prepared at postnatal days 4-5 from rat dentate gyrus and studied using the whole-cell patch-clamp technique. These cells expressed a fast-inactivating, 0.5 microM tetrodotoxin-sensitive Na+ current; a high-voltage-activated (HVA) Ca2+ current, which was 30 microM Cd(2+)- and partially 2 microM nicardipine-sensitive; and an inward rectifier current, which was sensitive to extracellularly applied 1 mM Cs+. The outward current pattern was composed of a delayed rectifier-like outward current sensitive to 20 mM tetraethylammonium (TEA) and a fast-inactivating, Ca(2+)-dependent outward current. This transient Ca(2+)-dependent K+ outward current was identified by a subtraction procedure. K+ currents recorded under conditions of blocked Ca2+ currents (after rundown of the HVA Ca2+ current or blocked by extracellularly applied Cd2+) were subtracted from control currents. By comparison with the current pattern of identified dentate granule cells, it is concluded that the investigated cell type originated from interneurones or projection neurones of the dentate hilus.     

Non-isothermal steady-state diffusion within Glaser’s condensation model

Non-isothermal diffusion models for water vapor transfer through building materials have been examined and their results compared with isothermal models. The Glaser condensation scheme with incorporated non-isothermal diffusion has been presented and its issues studied.