Hall–Petch relationship in the nanocrystalline selenium prepared by crystallization from the amorphous state

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Abstract
Bulk nanocrystalline selenium (nc‐Se) specimens with various grain sizes were prepared by isothermally crystallizing as‐quenched amorphous selenium. Microhardness measurements indicated that with a decrease in grain size, from 25 to 9 nm, hardnesses of nc‐Se samples increase dramatically from 0.34 to 0.98 GPa, i.e., a normal Hall–Petch relationship is obtainable over the grain size range studied for nc‐Se samples. Structural characterization of nc‐Se using x‐ray diffraction revealed an evident lattice distortion in the nanocrystalline state, namely the lattice parameter a is increased, whereas c is decreased simultaneously with a reduction of grain size. The variation tendency of microhardness with grain size can be well correlated with the change of the unit cell volume in nc‐Se, which implies that the lattice structure of the nanocrystalline state is a factor affecting the mechanical properties.