Effects of size reduction on the structure and magnetic properties of core-shell Ni_3Si/silica nanoparticles prepared by electrochemical synthesis

摘要

Nanostructured nickel silicides find application in electronics, high-temperature alloys, electrode materials and catalysis. In this work, the effect of size reduction on the structure and magnetic properties of β_1-Ni_3Si intermetallic phase nanoparticles is studied. Electrochemical selective phase dissolution (ESPD) was used to produce the β_1-Ni_3Si nanoparticles of different sizes (from 20 to 215 nm) by extracting β_1 nano-size precipitates from two-phase Ni-Si and Ni-Si-Al precursor alloys. The extracted nanoparticles have a core-shell structure with β_1-Ni_3Si core and an amorphous silica shell. Particles size and shape are controlled by the composition and thermal treatment of the precursor alloys. Precipitates size is scaled without modifying the ordered L1_2 lattice structure. The bulk β_1-Ni_3Si is ferromagnetic below 260 K with low saturation magnetization (2 emu/g), while the core-shell Ni_3Si/silica nanoparticles are superparamagnetic at low temperatures (<9-11 K) with low coercivity (<90 Oe) and magnetization >20 emu/g at 5 T. It is suggested that weak particle magnetic moments and low magnetic anisotropy of the L1_2 structure are responsible for these properties. The shell on one hand protects the core from degradation; however the oxidation of the core/shell interface region can influence the magnetic behavior of the nano-powders.