Dielectric switching above a critical frequency occured in iron mullite composites used as an electronic substrate

Highly crystallized mullite is synthesized via sol-gel technique in presence of iron of different concentrations at different temperature. FTIR spectroscopy, XRD and FESEM characterization showed prominent mullite phase which is found to depend on the concentrations of Fe2+ ions. The phase evolution and electrical properties like dielectric constant, tangent loss, a.c. conductivity and resistivity of the composites with increasing concentration of the doped metal at different temperature have been studied. The results showed a decrease in mullite phase up to 1.2 (M) which is mainly responsible for the increasing dielectric behavior with respect to undoped mullite. The hematite phase continued to increase with increasing doping concentration. The dielectric constant decreased with frequency for all the samples attaining constancy at higher frequency, which is normal behavior for dielectric ceramics. A.c. conductivity increased with frequency following Jonscher's power law and was found to depend on the amount of glassy phase and concentration of mobile ions present in the composites. The composite showed a minimum dielectric constant of 33.69 at 1.2 (M) concentration of iron at 2 MHz. Thus due to their good dielectric response the composite may widely be used use as electronic substrates like packaging materials for integrated high speed circuits, multilayer capacitors, high voltage insulators etc.