Melting Of Iron Nanoparticles Embedded In Silica

Melting of metallic nanoparticles and its size dependence are of fundamental interests in materials science and condensed matter physics. For decades, experimental studies on the size-dependent melting of metals are regretfully limited to some eight archetypal examples namely low-melting point metals In, Sn, Pb, Bi, Cd, Al and noble metals Au and Ag.In this work, to expand the above slim range of materials, the melting behavior of Fe nanoparticles embedded in SiO2prepared by using mechanical milling are investigated. Effects of factors in sample preparation on the size, isolation and thermal stability of Fe nanoparticles are systematically studied. On this basis, the size-dependent melting of Fe is successfully traced. In addition, preliminary efforts are carried out on the synthesis and melting behaviors of Cu and Ag nanoparticles embedded in SiO2.(i) By direct milling of Fe and SiO2powders, Fe nanoparticles were obtained which were in principle single crystals with a narrow size distribution and are dispersedly embedded in the SiO2matrix.(ii) For the milling at250rpm, for samples with different Fe-SiO2volume ratios, the particle size of Fe decreases gradually with extending milling time and finally becomes saturated at around14h. With the Fe-SiO2volume ratio increasing from1:7to6:4, the final particle size of Fe decreased from around16nm to9nm. For a same Fe content (Fe-SiO2volume ratio2:8) milled for16h, when the rotational speed increased from100rpm to300rpm, the size of Fe nanoparticles decreased from about33nm to13nm. However, for the2:8sample milled at200rpm for16h, when the milling was extended to50h, the particle size of Fe further decreased from22nm to14nm, indicating that the milling process needed a longer time to become complete if the rotational speed was less than200rpm.(iii) Results from thermal analyses showed that good isolation of Fe nanoparticles may be obtained by increasing the volume ratio of SiO2and by extending milling time until the size of Fe nanoparticles became saturated. For Fe nanoparticles with a diameter of about15nm, the melting point depression is30℃in comparison with bulk Fe, in accordance with our recent theoretical calculations.