Search

We re-examine the hadronic loop effects to the masses of $D^*_0$ and $D^*_{s0}$ calculated in quark models in the framework of heavy meson chiral perturbation theory (HMCHPT). The inaccuracy in the choice of the argument of the chiral loop functions in previous works is corrected. Our calculations consider the full one-loop corrections that appear at leading order in the chiral expansion of the effective Lagrangian. Unlike previous approaches, ours leads to satisfactory results in explaining the low mass of the observed scalar charm states reported by the Particle Data Group (PDG). It is found that the mass shift of bare $D^*_{s0}$ ($D^*_{0}$) state is mainly due to the $DK$ ($D\pi$) loop corrections for values of the couplings that are compatible with the measured ones. We show why previous approaches of using HMCHPT in studying mass shift effects due to chiral loops gave unsatisfactory results.

The extent of the cell voltage hysteresis observed in polarization curves of PEM fuel cells and the origins causing this effect are investigated by performing experimental polarization curves with different dwell times for a 50 cm2 PEM fuel cell. Electrochemical Impedance Spectroscopy measurements are carried out in order to determine the relative contributions of the different polarizations (ohmic, activation, and concentration polarization). Equivalent circuits were obtained from the impedance spectra, where the interpretation of the circuit parameters enabled the analysis of the origin and extent of the cell voltage hysteresis. It was identified that the cause of the cell voltage hysteresis is actually dominated by the changes in the concentration polarization at low current densities, and by the changes in the activation polarization at high current densities. The cell ohmic resistance (and consequently the membrane hydration) presents a minor effect on the cell voltage hysteresis. The effect of the dwell time in the cell voltage hysteresis (with values ranging from 15 to 25 mV) presents a mixed trend, with an initial decrease for increasing dwell times from 120 to 600 s, and a later increase and a final almost constant hysteresis for longer dwell times.