Conductor-insulator Composites Research Articles

A generalized differential effective medium theory

A generalization of the Differential Effective Medium approximation (DEM) is discussed. The new scheme is applied to the estimation of the effective permittivity of a two phase dielectric composite. Ordinary DEM corresponds to a realizable microgeometry in which the composite is built up incrementally through a process of homogenization, with one phase always in dilute suspension and the other phase associated with the percolating backbone. The generalization of DEM assumes a third phase which acts as a backbone. The other two phases are progressively added to the backbone such that each addition is in an effectively homogeneous medium. A canonical ordinary differential equation is derived which describes the change in material properties as a function of the volume concentration φ of the added phases in the composite. As φ→ 1, the Effective Medium Approximation (EMA) is obtained. For φ < 1, the result depends upon the backbone and the mixture path that is followed. The approach to EMA for φ ≊ 1 is analysed and a generalization of Archie's law for conductor-insulator composites is described. The conductivity mimics EMA above the percolation threshold and DEM as the conducting phase vanishes.

Dielectric constant of silver-thermosetting polyester composites

The dielectric constant of a conductor-insulator composite is measured and no percolation threshold is observed up toV=45%, whereV is the volume fraction of conducting filler. The composite is fabricated by dispersing silver-coated glass spheres in unsaturated polyester. It is found that the dielectric constant varies smoothly as (1−V)−3 within the experimental range and that this relationship may be satisfactorily interpreted on the basis of classical electromagnetic theory.

Charging phenomena in the scanning electron microscopy of conductor-insulator composites: A tool for composite structural analysis

Useful applications of charging phenomena occuring during scanning electron microscopy (SEM) of conductor-insulator composites have been investigated. Unlike the charging of insulating particles in conventional SEM techniques, the local field effect in a conductive composite enhances the relative secondary electron emission in the isolated conductive grains. This ‘‘reverse’’ charging characteristic was utilized for the mapping of dispersion, orientation and segregation characterization in conductor-insulator composite systems. The charging phenomenon directly reflects the relative electrical continuity of the conductive filler particles in the insulating matrix. The photographic display of the conductive filler arrangement in the composite using this charging phenomenon is termed a SEM charging micrograph. Carbon black/polyvinyl chloride composites with both spherical and chain-like carbon blacks were used in this study. The structural aspects of such composites as revealed by charge display techniques were found to be directly correlatable with the electrical properties of the composites. This technique is applicable to composites with both small (100 Å) and large (over 10 μ) fillers.

Charging phenomenon in conductor-insulator composites as displayed by the scanning electron microscope

Charging properties in conductor-insulator composites have been studied with the scanning electron microscope. Use of this instrument as a charging and display device demonstrated its usefulness as an analytical technique for describing important properties of composite structures, such as filler dispersion, segregation, orientation, and electrical continuity. These scanning electron microscope charging micrographs can be used to characterize composites with a wide range of particle sizes (100 Å to over 10 μm).

Studies on electrical conductivity of insulator-conductor composites

The variation of electrical resistivity of an insulator-conductor composite, namely, wax-graphite composite, with parameters such as volume fraction, grain size, and temperature has been studied. A model is proposed to explain the observed variations, which assumes that the texture of the composite consists of insulator granules coated with conducting particles. The resistivity of these materials is controlled mainly by the contact resistance between the conducting particles and the number of contacts each particle has with its neighbors. The variation of resistivity with temperature has also been explained with the help of this model and it is attributed to the change in contact area.