Ralf Strümpler

Nonlinear dielectric composites

Polymer composites with high dielectric constant are widely used as shielding and field grading materials. An improvement of the refractive grading can be expected for a dielectric constant being a nonlinear function of the electric field or the temperature, as known for nonlinear resistive field grading. An increase of the dielectric constant in those areas where the highest electrical field occur will result in a homogenization of the field distribution. Such composite materials can be considered as having the smart functions of sensing and actuating. The influence of nonlinear properties of filler material on the resulting dielectric properties has been studied both theoretically and experimentally. Calculations using effective medium theory show how much of the nonlinearity of the filler is transferred to the composite. They are compared with experiments on composites containing ferroelectric, semiconducting and varistor-type filler material in a thermoset or thermoplastic matrix. Depending on the filler type, the dielectric constant increases by a factor of up to three, for example, on raising the temperature from 30 degrees C to 150 degrees C. Such an enhancement can be sufficient to rearrange the field distribution in stressed insulating parts.

New low-voltage varistor composites

New varistor-type polymer composites for low-voltage application have been developed. The filler is made of commercially available doped ZnO-varistor powder. The polycrystalline filler particles act as varistors due to their typical grain-boundary structure. The presented varistor composite materials show very low values for the breakdown field strength down to 200 V mm−1, as compared with already existing varistor-type composites, and fairly highα-values in the range of 10.

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.

The influence of filler properties on the strong PTC effect of resistivity in polymer based conducting composites

Polymer based composites are very attractive materials for a plurality of technical applications. For electrical purposes the resistivity of such materials can be tuned over many orders of magnitude from highly insulating (1014&cm) to well conducting (10-3&cm) states. One particular class of polymer based composites even show a strong non-linear reversible change in resistivity with temperature between conduction and insulation. Such a pronounced effect of positive temperature coefficient of resistivity (PTCR) can be technically used, for example, in self regulating heating devices, for temperature sensing or, as a very challenging and important task, for current (i.e. over- as well as short-circuit currents) limiting and interrupting devices. The PTCR-devices act similar like fuses but repetitively, which offers technical and economical benefits. In the present work we investigate how the physical properties of the conducting filler influence the switching characteristics of the PTC material. Experimental results on resistivity and its change under active heating by Joules losses during current flow are presented and discussed for different composites, compounded with fillers like carbon black, Ni/Ag, TiB2 or WC/Co. The strong resistance change caused by break-off and separation of particle-particle micro-contacts is driven by the very different thermal expansion coefficients of filler and matrix. It is in particular demonstrated how the heat capacity of filler-particles influences the dynamics of the micro-contact separation.