CHANNELING STUDY OF MELTING AND SOLIDIFICATION OF LEAD NANOCRYSTALS IN ALUMINUM

Nanometer-sized Pb crystallites were obtained in Al single crystals by 423 K implantation of 150 keV Pb+ ions with a fluence of 2.3 X 10(16) cm(-2). The crystallites grow in perfect topotactical alignment with the matrix with a cube/cube orientation relationship, Lead depth profi les were obtained using the Rutherford backscattering (RES) technique. With the RES and channeling analysis two major distributions of nanoc rystals were observed. The first one with the average size of crystall ites of about 13 nm is located within depth region 30-65 nm, acid the second distribution (70-105 nm) has the average size of crystallites o f 9.9 nm. Measurements of melting/solidification of Pb nanocrystals we re performed with the channeling technique. A thermal hysteresis for c rystallites as well as for channeling in Al matrix was observed. The n anocrystals show large superheating (similar to 75 K above the bulk me lting point of Pb) as well as supercooling (similar to 35 K) during th e heating cycle. The size dependence of melting of the crystallites is deduced from the measurements using Monte Carlo channeling simulation s. These results are compared with those obtained by transmission elec tron microscopy (TEM). They are discussed in a phenomenological contex t considering the lack of free surfaces and a reduction of thermal vib rations for surface atoms in topotactical nanocrystals.