New composites of ZnO–P2O5/Ni having PTC transition and high Seebeck coefficient

A bstract In this article, we report the electrical conductivity (σ) and Seebeck coefficient (S) of ZnO–P2O5 matrix filled with conductive powder of nickel (Ni). The variation of σ versus volume fraction of Ni showed a non-conducting to conducting phase transition at percolation threshold (28 vol. %). The change of S from high positive to negative values exhibits that this transition is accompanied by the passing of carrier charge from p to n type. On the other hand, the measurements of σ and S as function of temperature, above the percolation threshold, showed a positive temperature coefficient (PTC) phase transition at Tc ≥ 400 K, linked with a high S = − 5000 μV/K, giving highest power factor PF = σ.S 2≈ 2.10− 4 W m− 1 K− 2. The temperature dependence of the volume expansion enabled to confirm that this transition is associated to the thermal volume variation in matrix. However, the temperature dependence of σ below the percolation threshold showed two different mechanisms: thermally activated hopping behavior at high temperatures and Mott's variable range hopping (VRH) at low temperatures.