Size-dependent thermopower in nanocrystalline nickel

Thermopower (S-n) of compacted nanocrystalline Ni of average particle size D=38, 32, and 25 nm at low temperatures (5-300 K) is reported here. S-n exhibits striking deviations from bulk thermopower S-bulk behavior revealing evolution of S-n with change in D. As D decreases, there is significant decrease in S-n, gradual disappearance of phonon drag minimum and sign change to positive below 38 K compared to usual negative S-bulk. They have been attributed to electron and phonon scattering at grain boundaries and phonon confinement. Small minima observed near 25 K in thermopower difference Delta S are correlated with superparamagnetic transitions. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3167302]