Phase transformations in Cd–Ni nanostructured system at elevated temperatures

Resumo

Using X-ray diffraction (both in vacuum and in air), derivatomass-spectrometry and thermo-programmed oxidation, phase transformations in nanostructured Cd–Ni system occurring while heating up 720 °C were described. These metal powders were obtained by joint reduction from aqueous solutions of salts with hydrazine. When study their composition, earlier unknown phases were detected (intermetallide CdNi₃ and solid solution Cd_xNi_1–x with face centered cubic structure) and among the known intermetallides (Cd₅Ni and CdNi), only Cd₅Ni was found. As a consequence, phase transformations in the obtained nanostructured powders were remarkably different from those present on the Cd–Ni phase diagram. The intermediates and FCC phase decompose independently with formation of individual initial components (Cd and Ni) without intermediate product. At relatively high Cd concentration in the system, intermediate CdNi₃ phase formation from Cd₅Ni and Cd_xNi_1–x phases, which are polar opposites in composition, is possible. The decomposition temperatures of Cd₅Ni and melting point of Cd in the nanostructured state were lower than those specified by the phase diagram by 170 and 70 °C, respectively, which well corresponded to the concept of effective (high) temperatures attributed to the nanostructured systems due to the energy saturation of the con stituent nanocrystallites.