The dielectric and piezoelectric behaviour of pyrolusite (polycrystalline ore of MnO²)

Abstract

The variation of dielectric constantɛ of pyrolusite (sintered polycrystalline ore of MnO₂) with temperature, frequency and voltage has been studied within the frequency range 200 c/s to 10 kc/s.

The samples studied show a very high value of dielectric constant ≈ 10⁵. Theɛ-temperature curve indicates thatɛ attains a maximum value at a certain temperature. The temperature at whichɛ attains a maximum value appears to vary from sample to sample. At higher temperatures (i.e. higher than that at whichɛ attains a maximum value),ɛ decreases with temperature asɛ=c/(T−Θ).

The dielectric constant is dependent on the frequency; it has a higher value at lower frequencies. The applied voltage across the specimen has a marked influence on the dielectric constant, which increases rapidly with voltage. A capacity with pyrolusite as dielectric shows a hysteresis loop (electrical). The dielectric loss factor tanδ is found to depend on temperature, frequency and voltage. Tanδ attains a maximum value at lower range of frequencies between 300 c/s to 700 c/s, for a given voltage, and temperature. This indicates the existence of dielectric relaxation for these specimens. Tanδ also varies exponentially with temperature.

Samples of pyrolusite studied show a piezo electric effect as well. The electromechanical coupling coefficient is found to vary from 2·4×10⁻⁵ to 8·8×10⁻⁵ e.s.u/ dyne. Slabs cut from the samples of pyrolusite mineral were found to control the frequency of a Hartley oscillatory circuit. The resonant frequency range was found to increase as the thickness of the slab is reduced.