Thermodynamic stability of nanocrystalline palladium

The chemical potentials of nanocrystalline palladium have been determined by EMF-measurements at a temperatures of 613 K. The experiments show the thermodynamic instability of nanostructured Pd. From the time dependence of the EMF, it can be concluded that the nanocrystalline material relaxes on two time scales towards the thermodynamic equilibrium. The faster one is related to the structural relaxation of the grain boundaries whereas the slower one is related to grain growth. As a result of the instability of the nanocrystalline grain boundary structure the properties of the grain boundaries and therefore also those of the nanocrystalline material depend on the preparation technique and the subsequent annealing treatment. Consequently, grain boundaries formed under highly non-equilibrium conditions, such as compaction of nanosized particles, by vapour condensation techniques or by mechanical milling of elemental or alloyed powders are most likely to be structurally (and chemically) different. This should be considered if the properties of nanocrystalline materials formed by different processes are compared.