PNNL

Abstract

The anelastic spectrum of ammonia borane (NH3BH3) was measured in the temperature range between 1.3 and 300 K. The structural phase transition near 220 K is accompanied by a substantial variation of the elastic modulus (25-30 %) and a spike of the elastic energy dissipation. The order-disorder phase transformation which occurs with a minute hysteresis, 0.4 K between heating and cooling is indicative of a first order phase transformations with a narrow region of coexistence between the two phases. The kinetics of the transition was investigated in detail. In addition, a thermally activated relaxation peak is revealed around 100 K for a sample vibration frequency of ~ 1 kHz, which is attributed to the dynamics of the rotations and torsions of the NH3-BH3 groups. Finally, the time-dependent energy loss on heating after ageing the sample below the structural phase transformation temperature was interpreted in terms of annealing of dislocations