Colossal dielectric permittivity in Co-doped ZnO ceramics prepared by a pressure-less sintering method.

Dielectric properties and impedance spectroscopic studies of single phase Zn1- xCoxO (0 ≤ x ≤ 0.05) ceramics, synthesized by a pressure-less solid state reaction method, have been carried out to investigate the origin of colossal dielectric permittivity (CP), ε' ∼ 105, in a wide frequency (2 × 101-2 × 106 Hz) range. These results show that a defect density within the grain is present in the materials due to the use of pressure-less sintering at high temperature for a long duration of time. The colossal dielectric response is essentially due to this electronic inhomogeneous conduction mechanism within the material due to the production of absorption current in the thin grain boundary region which accumulates charge at the interface and induces Maxwell-Wagner interfacial polarization. Moreover, this defect structure further increases with the addition of Co ions and enhances the CP property. An effective way to control the colossal dielectric properties is to control the defect density within a grain by using a proper sintering method.

The effect of Y3+ substitution on the structural, optical band-gap, and magnetic properties of cobalt ferrite nanoparticles.

In this study we investigated the structural, optical band-gap, and magnetic properties of CoYxFe2-xO4 (0 ≤ x ≤ 0.04) nanoparticles (NPs) synthesized using a combustion reaction method without the need for subsequent heat treatment or the calcing process. The particle size measured from X-ray diffraction (XRD) patterns and transmission electron microscope (TEM) images confirms the nanostructural character in the range of 16-36 nm. The optical band-gap (Eg) values increase with the Y3+ ion (x) concentration being 3.30 and 3.58 eV for x = 0 and x = 0.04, respectively. The presence of yttrium in the cobalt ferrite (Y-doped cobalt ferrite) structure affects the magnetic properties. For instance, the saturation magnetization, Ms and remanent magnetization, Mr, decrease from 69 emu g-1 to 33 and 28 to 12 emu g-1 for x = 0 and x = 0.04, respectively. On the other hand the coercivity, Hc, increases from 1100 to 1900 Oe for x = 0 and x = 0.04 at room temperature. Also we found that Ms, Mr, and Hc decreased with increasing temperature up to 773 K. The cubic magnetocrystalline constant, K1, determined by using the "law of approach" (LA) to saturation decreases with Y3+ ion concentration and temperature. K1 values for x = 0 (x = 0.04) were 3.3 × 106 erg cm-3 (2.0 × 106 erg cm-3) and 0.4 × 106 erg cm-3 (0.3 × 106 erg cm-3) at 300 K and 773 K, respectively. The results were discussed in terms of inter-particle interactions induced by thermal fluctuations, and Co2+ ion distribution over tetrahedral A-sites and octahedral B-sites of the spinel structure due to Y3+ ion substitution.