Hydrogen desorption kinetics of hafnium hydride powders

The kinetics of hydrogen gas release from hafnium hydride are investigated by combining experiments and density functional theory. The material is a candidate neutron moderator and attenuator for compact nuclear reactors, where hydrogen release will lead to a degradation in moderating function. Experimentally, we have studied the decomposition of epsilon phase (HfH2-x) powders from 25-1000°C using thermogravimetry and X-ray diffraction. Isochronal heating reveals 3 characteristic desorption regions corresponding to the release of hydrogen from each phase (ε-HfH2-x, δ-HfH1.6-x and α-Hf), at ∼ 350, 415, and 700°C. These results are well supported by the modelling output from density functional theory. A Kissinger analysis allowed for activation energies for desorption to be calculated (∼150 kJ/mol, 170 kJ/mol and 90 kJ/mol respectively). The peak shape and desorption rate data suggests that a second order diffusion limited reaction controls the ε→ε+δ desorption, a first order interface limited reaction controls the ε+δ→δ reaction, and a surface limited zeroth order reaction limits the complete desorption of the δ+α phase. The analysis suggests that, at least for δ→α regime, engineering solutions for improved thermal stability should focus on reductions in surface reactivity.