Low-temperature electric and thermoelectric properties of Ni/Al ₂ O ₃ granular films

Low-temperature electric and thermoelectric properties of granular films (GF) of (Ni)x/(Al2O3)100-x with 13 ≤ x ≤ 54 at.%, prepared by EB-PVD, have been studied. In the temperature range 77 - 290 K, it was observed that the resistivity ρ(T) exhibited an exp(T0/T)n law in the dielectric regime, with n ≃ 1/2, where T0 is a characteristic temperature. With increasing x, the variation of resistivity with temperature changed to an activated form ρ(T) ∼ exp(Ear/kT) near a percolation threshold and then to a simple metallic behaviour ρ(T) ∼ T. We have extracted values of the estimated tunnelling probability decay length (ξ) and the optimal hopping distances between nanoparticles (r0). A significant increase in thermopower (from −17.5 μV/K at 280 K to −1331 μV/K at 100 K) with decreasing temperature was observed for a sample with 13 at.% Ni.