Felix Greuter

Nonlinear resistive electric field grading Part 2: Materials and applications

The article emphasizes the ability of new nonlinear resistive materials to be tuned for specific applications, which opens up new possibilities in designing electric field control of products for medium and high voltage applications.

Nonlinear resistive electric field grading part 1: Theory and simulation

Nonlinear resistive field grading permits the optimization of designs of ac and dc accessories in medium and high voltage systems. This first article of two introduces the theoretical basics required for a successful application of this modern technology.

Transient response of electrically active grain boundaries in polycrystalline semiconductors

In many polycrystalline semiconductors, the current transport is controlled by potential barriers formed at the grain boundaries. We investigate the response of ZnO grain boundaries to high-voltage pulses with risetime of less than 1 μs. The experimental results, in particular the voltage overshoot effect, cannot be explained by the dynamics of electrons (majority carriers) alone. A model which includes holes (generated by hot electrons in the depletion regions) yields an appreciable overshoot when the steady state is close to a current-voltage bistability. In this case, the approach to equilibrium is altered by the delayed response of the barriers to the presence of holes. Further implications of the model are also discussed.

Correlation of Electrical, Dielectric And Mechanical Properties of Polymer Composites

The electrical and dielectric response upon thermal expansion and contraction of polymer composites have been studied for thermoset polymers containing ceramic or metallic fillers. Before curing, the resistivity p of epoxy containing a metallic filler is in the range of 10 6 to 10 7 Ωcm. During curing a sudden reduction of p by eight orders of magnitude is observed. The shrinkage of the polymer during cross-linking and the build-up of internal stresses results in a lower interparticle contact resistance. The contraction can be reversed by heating. Around curing temperature, the resistivity increases by six to eight orders of magnitude due to the expansion of the material. The resistivity increases progressively until the percolation network of the conducting particles is interrupted and the composite reaches a high-ohmic state. Surprisingly, the critical temperature is related to the curing temperature and not to the glass transition temperature, as would be generally expected. Changes of the interparticle gap size can also be observed in the dielectric constant e. For conducting fillers covered by a thin oxide layer, e decreases by several orders of magnitude after a certain expansion has been achieved. However, for poorly conducting or insulating fillers dielectric response becomes insensitive to the gap distance. Hence, the electrical or dielectric response of polymers containing metallic fillers can be used as a sensitive probe for the internal stress state and the corresponding micro-structure of the composite.

Varistor Ceramic: Residual Strain and Texture Analysis by Neutron Time-of-Flight Diffraction

Strain/stress experiments for characterization of micro- and macro-strain distribution and texture experiments were carried out on a varistor ceramic using neutron-TOF-diffraction. By a radial strain scan, residual strain has been determined on the (1010 ), (0002), (1012), and (1120 ) Bragg reflections of the hexagonal ZnO as well as on the (400) reflections of the cubic spinel Zn7Sb2O12. Residual strain values in the range of -1.5x10-4 (contraction) and 4x10-4 (dilatation) were observed for ZnO, with indications of macro-strains across the cylindrical sample and substantial micro-strains between the different phases. The texture on both phases (ZnO, Zn7Sb2O12) is characterized by a weak, non-ideal fibre texture around the cylinder axis. The results indicate that the texture pattern might coincide with the strain. To our knowledge, these are the first texture and residual strain investigations in metal oxide varistor elements and they will be discussed in detail.

Current Flow and Structural Inhomogeneities in Nonlinear Materials

Nonlinear current flow in real 3-d composite materials is far more complex than suggested by usual brickstone models. We present experimental results on controlled seeding of structural inhomogeneities in varistor ceramics. In particular, we discuss the influence of mesoscopic imperfections on the characteristic electrical parameters. A main effect is a change in the statistical distribution of the grain size. As a consequence, the breakdown voltage drops very sensitively due to the disorder, while the shape of the I-V curve is hardly affected. We discuss also the extremely nonuniform filamentary current pattern, being illustrated by electroluminescence studies.