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

Abstract

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 (28vol. %). 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≥400K, linked with a high S=−5000μV/K, giving highest power factor PF=σ.S2≈2.10−4Wm−1K−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.