Positive temperature coefficient and high Seebeck coefficient in ZnO–P2O5/Co composites

Abstract

This article reports a study of electrical properties of new Zinc Phosphate glass/Cobalt composites (45mol.% ZnO–55mol.%P2O5) (ZP/Co). The measurements of electrical conductivity at room temperature as a function of cobalt's concentration showed a non-conducting to conducting phase transition at percolation threshold of 27vol.%. The Seebeck coefficient obtained under the same conditions, accompanies a sign, with high positive and negative values below and above the percolation threshold respectively, depicting a p- to n-type conducting phase transition, confirming the conductivity measurements. Then, the measurements of electrical conductivity and Seebeck coefficient above the percolation threshold as a function of temperature showed an original conducting to insulating phase transition, called Positive Temperature Coefficient (PTC) at T=420K, associated to a high negative value of S≤−8000μV/K, with the highest power factor PF=σS2≈8×10−3W m−1 K−2. The thermal measurements of volume expansion confirm this transition, indicating matrix dilation around this temperature. However, the thermal behavior of the electrical conductivity and Seebeck coefficient data obtained below the percolation threshold showed different mechanisms i.e.; Small Polaron Hopping (SPH) mechanism at high temperatures and Mott's Variable Range Hopping (VRH) at low temperatures.