Experimental and computational investigations of LaNi(5-x)Alx (x = 0,0.25, 0.5, 0.75 and 1.0) tritium-storage alloys

摘要

Although already scientists in recent years have reported some experimental and theoretical results of LaNi-Al series of tritium-storage alloys, several key aspects remain the subject of considerable debate. In an effort to interpret some of these unknowns, we have performed experimental and theoretical investigations for LaNi_(5-x)Al_x(x = 0, 0.25, 0.5, 0.75 and 1.0) tritium-storage alloys. Firstly, the XRD characterization indicates that the unit cell volumes of LaNi_(5-x)Al_x increase with Al content in alloys. Secondly, the PCisotherm measurement of LaNi_(5-x)Al_xalloys shows that their hydrogen absorption/desorption plateau pressures reduce with the increase of Al content while their plateau widths narrow simultaneously. The deuterium absorption/desorption plateaus have a similar trend to hydrogen's except for their plateaus being higher than hydrogen's. To explain the above experimental findings, a series of calculations based on density functional theory(DFT) and frozen phonon approach have been performed. The results manifest that:(1) the partial substitutions of Al for Ni reduce the hydrogen formation energies of LaNi_(5-x)Al_xH and the number of available interstitial sites, and therefore lead to the absorption/desorption plateau pressures being reduced and the plateau widths being narrowed down at the same experimental temperatures;(2) the covalent interaction between H and Ni is an important factor for estimating the stability of LaNi_(5-x)Al_x-H system;(3) since the calculated enthalpy change H is generally more accurate than the calculated entropy change S with respect to the corresponding experimental value for each LaNi_(5-x)Al_xH(or D), the curves of H vs. hydrogen storage capacity instead of Van't Hoff relation, can be used to predict the experimental plateau pressures of LaNi_(5-x)Al_x-H(D or T) at a given temperature;(4) the hydrogen isotope effect of LaNi_(5-x)Al_x-H(D or T) system can be quantitatively described as a linearity relation between ⊿ZPE + ⊿H^(vib) and 1/√mQ(Q = H, D, T). From the good agreement between the predicted and experimental ln(P_H/P_0) and ln(P_D/P_0), it is deduced that predicting ln(P_T/P_0) of LaNi_(5-x)Al_x T is feasible. The procedure of pre-computing and comparing curves of H vs. hydrogen storage capacity proposed in this paper provided an attractive tool to increase the efficiency of experimental alloying design of hydrogen(deuterium or tritium) storage materials.