SEM and TEM characterization of sodium alanate doped with TiCl3 or small Ti clusters (Ti13·6THF)

The evolution of the microstructure under desorption and absorption of hydrogen in sodium alanate doped with 5 mol.% Ti on the basis of either TiCl3 or Ti13·6THF is presented. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) have been applied to investigate ball-milled samples during and after various hydrogen desorption and absorption cycles. Upon the phase transitions the change in the microstructure is characterized by an increase in the porosity. The overall transformation is a random and irreversible process. After several cycles in the dehydrogenated state Al crystallites of 300 nm in size have been detected. Additionally, EDX analysis suggests a homogeneous distribution of Ti together with Na, Al and Cl. Moreover, at different stages of the reaction a pronounced variation of the Al concentration in the particle has been observed in contrast to all other elements. High-resolution transmission electron microscopy (HRTEM) combined with EDX analysis does not detect the presence of Ti particles implying that the particles size must be less than 0.8 nm.